Gun barrel

This is a division of application Ser. No. 08/753,182 filed Nov. 20, 1996, now issued as U.S. Pat. No. 5,856,631 on Jan. 5, 1999.

This invention was disclosed in part in earlier filed Provisional Patent Application No. 60/006,978 filed on Nov. 20, 1995, and Provisional Patent Application No. 60/010,750 filed on Jan. 29, 1996, each entitled “Gun Barrel”.

This invention relates to gun barrels, and particularly to a light weight, ultra-high strength, corrosion resistant gun barrel that is virtually burst-proof, and has low heat conductivity and low coefficient of friction with projectiles, hence heats more slowly than conventional barrels.

BACKGROUND OF THE INVENTION

Gun barrels have been made in substantially the same way since the early 1900's, with only minor improvements in processes and materials since then. The process is basically to mount a large cylindrical steel casting or forging for rotation about its axis and machine the outside to a tapered cylindrical barrel blank. The blank is then mounted in a gun drill and rotated about its axis against a drill.to bore axially through the barrel blank. Finally, a broaching operation cuts shallow helical grooves to form rifling between the grooves.

Mostly through trial and error, refinements have been made to manufacturing techniques for making gun barrels to correct for inaccuracies that were noted under certain conditions of use. For example, rapid or extensive firing of the gun heats the gun barrel, and it was found that the uniformity of the barrel thickness around the barrel is important to prevent unequal thermal expansion that can distort the barrel into a curved or even wavy shape and ruin the accuracy of the gun. To minimize this type of distortion, the barrels are turned as accurately as possible after the bore as been bored, and high accuracy guns are provided with thick walled barrels to minimize the effects of whatever variations in barrel wall thickness remain.

Differential thermal expansion is also believed to be responsible for non-uniform twisting of the barrel as it, heats during use, caused by the non-uniform thickness of the barrel wall due to the rifling in the bore. The slightly corkscrew shape of the barrel is also detrimental to accuracy of the gun.

The high temperature of the barrel is a consequence of high rate of fire and is considered to be inevitable. At present, the only known techniques to prevent high barrel temperature involve various types of active cooling, including the use of water jackets around the barrel. Little effort has been made to study the source of heat, which is primarily conduction from the burning propellant in the breech and the barrel, and also friction between the projectile and the bore. Reduction of this heat flux into the barrel would retard the rise in temperature of the barrel during use and alleviate some of the deleterious effects of the high temperature on barrel performance.

Conventional steel alloys used in gun barrels, including rifles, side arms, and shotguns as well as barrels for large naval and ground artillery and high rate-of-fire weapons such as machine guns and cannons are heat treatable to increase their strength. However, the trade-off for attaining high strength by heat treatment in steel alloys is an increase in brittleness. Put another way, the ability of the steel alloy to yield without rupturing when its yield strength is exceeded, a property known as toughness, is lost or reduced when the steel is heat treated to achieve high strength. High strength brittle material in a gun barrel is dangerous because overpressure caused by a plugged barrel or excessive powder loads, or weakness in the barrel caused by damage, fatigue, corrosion, or other such factors could cause the barrel to burst catastrophically instead of just bulge. Since the bursting usually occurs at the breech, near the shooter's face, the potential for serious injury, blinding, or death is high. Accordingly, it is the normal practice, although unfortunately not universal, for gun manufacturers to sacrifice potential strength and hardness of their barrel materials for toughness by not heat treating to maximum strength, usually less than 32 KSI. As a result, the barrel wall thickness must be made commensurably thicker and the soft condition of the barrel material is susceptible to rapid erosion from passage of the projectiles.

A goal in designing modern military weapons is to attain higher muzzle velocity for the projectile to attain longer range, flatter trajectory, higher impact energies and greater accuracy. One conventional technique for attaining higher muzzle velocity is to increase the barrel length to give a longer time during which the propellant gas pressure can act on and accelerate the projectile. Apart from cost, the primary limitation on barrel length is weight. The increased moment of inertia of a long barrel increases the load on the training mechanisms used to point the barrel, especially when tracking a moving target or shifting between targets in a rapidly evolving battlefield situation. Moreover, the vibration and resonant conditions are compounded in a long barrel.

Another technique for increasing the muzzle velocity is to increase the propellant energy. The limitations of this technique are the burst strength of the barrel, primarily in the breech area since the pressure spike of the reacting propellant occurs primarily while the projectile is near the breech. To flatten that pressure spike, the propellant may be adjusted to react more slowly and provide a more steady pressure against the projectile. However, the pressure pulse created by the muzzle blast from the propellant when the projectile exits the barrel must be controlled to prevent injury to personnel or equipment in the vicinity. Barrel materials that could withstand an extreme pressure pulse from a high energy propellant would enable a gun to greatly increase the muzzle velocity without creating a muzzle blast that exceeded the established safety limits.

Steel is a dense material, and gun barrels made of steel are heavy. The weight is increased even more because of the need to make the barrel wall thicker since it cannot be safely heat treated to maximum strength. The heavy barrel is a mere annoyance for hunters and recreational shooters, but it seriously impacts the capability of military systems that must be burdened by the great weight of conventional steel gun barrels. Aircraft must sacrifice load or range to carry the heavy guns using these barrels, reducing the quantity of ammunition the aircraft can carry. The swing weight of large naval guns becomes so great that the train and elevation drives of the guns become immense and slow. The strength needed to resist the high energy propellant loads necessary to achieve ultra-high velocities needed for long range, flat trajectory, high accuracy shooting are practically unattainable because of the great thickness of barrel wall needed, which makes the gun so heavy as to be unmanageable. Moreover, the soft condition of the barrels causes rapid wear of the bore, especially in rapid fire situations where the barrel gets very hot and loses even more of its already low strength. The resultant loss of accuracy of these military weapons make further expenditure of ammunition a total waste.

Corrosion resistance of high carbon steels is notoriously poor. Special coatings and other techniques ate available in great profusion to protect the gun barrels from corrosive influences such as salt water, most acids, products of propellant combustion, and many other substances common in the environment. However, most such coatings are most useful if applied frequently, especially immediately after each use of the gun, but it is rarely convenient to do so. Consequently, there is a period following use of the gun before it is cleaned and coated with the protective coating during which rapid corrosion can occur, especially since the combustion products of the propellant, and the projectile fragments remaining in the barrel can create galvanic corrosion. The resultant pitting of the bore then tends to trap additional corrosive materials, further exacerbating the corrosive effects. The effort to find barrel materials that can resist the effects of these corrosive substances has never produced a material that meets the other requirements for a gun barrel.

Vibration and shock of firing large caliber machine guns and artillery tend to be inimical to accuracy. The vibration must be allowed to damp out before the next round is fired or there would be little certainty where the gun will be pointed when the projectile leaves the muzzle. Shock transmitted through the barrel on initiation of the propellant charge may influence the interaction of the projectile in the bore, especially the reflected wave rebounding back from the muzzle. These vibration and shock waves may also interfere with the interaction of the barrel on its mounting structure, and also reduce the life of the gun by fatigue.

Hot plastic deformation of a conventional steel barrel is a serious problem, especially in military guns. At elevated temperatures, the steel barrel is effectively hot forged slightly each time the gun is fired, increasing the internal diameter of the bore slightly and, over time, increasing it enough that the bore, even without erosion, is no longer within bore tolerance. The projectile is loose in such an over-sized bore and has poor accuracy. Moreover, the blow-by of propellant gasses around the projectile in the bore is so great that the projectile does not develop the velocity it needs to attain its specified range, and instead falls short of its intended target.

Long gun barrels present special accuracy problems, especially large caliber guns on the order of 155 mm or larger with cantilevered barrels. Such guns require relatively thick-walled barrels to contain the high propellant gas pressure and provide a large heat sink to prolong the period during which high rate-of-fire can be tolerated before the accuracy deteriorates to the point beyond which further expenditure of ammunition is useless. Such conventional steel thick walled gun barrels are very heavy and have a tendency to droop at the muzzle end when trained at low elevations, especially when the barrel becomes hot and the Young's modulus of the steel drops. These have been intractable problems in the past because of the need for high burst strength and the high density of the only know materials that were proven for use in gun barrels. A composite metal gun barrel that is comparatively light weight, has a high Young's modulus for stiffness, and a high burst strength would be a very welcome development, especially for large caliber guns.

Attempts have been made for years to produce composite gun barrels, always without practical success. The materials used are usually very expensive and labor intensive to build into a barrel. More seriously, however, environmental and service conditions have a destructive effect on composite barrels and no satisfactory solutions to these problems have been developed. The problems include a mismatch of coefficients of thermal expansion between the several elements in the composite barrel, resulting in poor mechanical coupling between those elements and insufficient compressive preload. The attempts to correct these problems are complex and impractical in a production environment. The composite elements tend to be brittle, shock sensitive and vulnerable to attack by common environmental substances such as salt water, as well as acids, hydraulic fluid and other substances common around guns, especially on naval vessels.

Thus, for many years there has been a serious need for a gun barrel, made of tough, high strength materials, that is relatively light weight so that the gun barrel may be made thinner than current barrels and the thin barrel combined with the low density material substantially reduces the weight of the barrel. The high strength and toughness of the barrel materials would permit use of higher energy propellant loads for increased muzzle velocity, range and accuracy. Ideally, the gun barrel would be self damping and immune to the effects of salt water, acids, and the corrosive combustion products of the projectile propellant. Finally, such an ideal gun barrel would have a low coefficient of friction with the projectile materials, a high heat capacity, and low coefficient of thermal expansion to minimize the distorting effects of differential thermal expansion.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improved light weight gun barrel that is tough, strong and corrosion resistant. Another object of the invention is to provide a process of manufacturing a gun barrel from a low density, tough, high strength, corrosion resistant material Yet another object of this invention is to provide a composite barrel having elements tailored specifically to provide bore erosion resistance, damping, and high bursting strength.

These and other objects are attained in a gun barrel that is made of a nickel-titanium alloy or intermetallic compound such as Nitinol. The barrel may have a barrel liner tube of one nickel-titanium alloy or intermetallic compound such as 60 Nitinol or a low transition temperature nickel-titanium composition in its austenitic state, and may be prestressed in compression by a sleeve of the same or another nickel-titanium intermetallic compound or alloy. An intermediate sleeve of 55 Nitinol may be used to provide integral damping to absorb shock and vibration to prevent barrel whipping and other undesired off-axis deflection.

DESCRIPTION OF THE DRAWINGS

The invention and its many attendant objects and advantages will become more clear upon reading the following detailed description of the preferred embodiment in conjunction with the following drawings, wherein:

FIG. 1

is a sectional elevation of a gun barrel according to this invention secured to a conventional breech by a Nitinol coupling;

FIG. 2

is a sectional elevation of a gun barrel according to this invention having an integral breech;

FIG. 3

is a sectional elevation of a shotgun barrel made in accordance with this invention;

FIG. 4

is a schematic drawing an electrical discharge machine adapted for drilling barrel bores and sleeves;

FIG. 5

is a schematic drawing of the EDM machine shown in

FIG. 4

, after drilling the barrel bore;

FIG. 6

is a sectional elevation of a second embodiment of a gun barrel made in accordance with this invention;

FIGS. 7-9

are schematic diagrams of a punch process for hot forming an axial bore in a Nitinol billet;

FIG. 10

is a sectional elevation of a third embodiment of the invention having an Austenitic Nitinol outer sleeve and a Martensite Nitinol inner sleeve around a barrel liner of 60 Nitinol;

FIG. 11

is a sectional elevation of a fourth embodiment of a gun barrel according to this invention;

FIG. 12

is a sectional elevation of a fifth embodiment gun barrel according to this invention; and

FIG. 13

is a sectional elevation of another form of the fifth embodiment of the invention according to this invention;

FIG. 14

is a sectional elevation of yet a third form of the fifth embodiment of a gun barrel in accordance with this invention;

FIG. 15

a sectional elevation of a sixth embodiment of the invention made in accordance with this invention.

FIG. 16

is a sectional elevation of a seventh embodiment of the invention made in accordance with.this invention, having a steel outer tube with a Nitinol liner'sleeve;

FIG. 17

is an end elevation of, the gun barrel along lines

17

—

17

in

FIG. 16

;

FIG. 18

is a schematic view of a press apparatus for pressing the Nitinol liner of

FIG. 16

into the steel outer tube;

FIGS. 19 and 20

are sectional elevations of a facility for manufacturing the gun barrel shown in

FIG. 16

using the shape memory characteristic of the Nitinol liner sleeve material;

FIG. 21

is a sectional elevation of the liner sleeve produced by the facility shown in FIG.

19

and/or

FIG. 20

positioned within a steel outer tube and ready for shape-memory expansion to produce a permanent bimetallic gun barrel having a steel outer tube and a Nitinol liner sleeve.

FIG. 22

is an exploded sectional elevation of a second form of the seventh embodiment of the gun barrel according to this invention, showing the Nitinol liner sleeve retained within the steel outer tube by a threaded retainer cap;

FIG. 23

is a sectional elevation of a gun barrel assembled from the exploded elements shown in

FIG. 21

;

FIGS. 24-26

are sectional elevations of alternate configurations of gun barrels having Nitinol liner sleeves retained by threaded retainer caps in steel outer tubes;

FIG. 27

is a sectional side elevation of a section of another form of the embodiment shown in

FIG. 16

, showing the Nitinol liner sleeve attached to the steel outer tube of the gun barrel by a compression clamp;

FIG. 28

is an end elevation of the gun barrel section shown in

FIG. 27

;

FIG. 29

is an enlarged sectional end elevation of a portion of the two clamp halves shown in

FIG. 28

;

FIG. 30

is a sectional elevation on a plane perpendicular to the bore of another embodiment of a gun barrel made in accordance with this invention;

FIG. 31

is a cross section of barrel liner segments assembled onto a mandrel and inserted into a barrel tube;

FIG. 32

is a cross section of one of the barrel liner segments shown in

FIGS. 30 and 31

;

FIG. 33

is a cross section of a barrel liner piece before forming into the cylindrical form shown in

FIG. 32

; and

FIG. 34

is an elevation, partly in section of the liner piece shown in

FIG. 33

being formed in a forming die.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, wherein like reference numerals designate identical or corresponding parts, and more particularly to

FIG. 1

thereof, a barrel

30

is coupled to a breech

32

by a connector

34

. The breech

32

may be a conventional steel structure machined from conventional gun materials such as 4140 steel or 416 stainless steel. The barrel

30

is an elongated tube

36

made of Type 60 Nitinol bored axially completely through from the breech end

38

to the muzzle end

40

. A series of wide, shallow helical grooves are cut into the bore

42

of the tube

36

, leaving helical lands

44

between the grooves, constituting rifling by which the bullet

46

is spin stabilized as it is propelled through the bore

42

. The wall thickness of the barrel tube

36

may be uniform as shown in

FIG. 1

, or may be tapered toward the muzzle end

40

to accommodate the higher propellant pressures at the breech end when a cartridge

48

is fired in the breech.

The connector

34

is an annular ring of shape memory Nitinol such as Type 56 Nitinol. Type 56 Nitinol is an intermetallic compound of 56% nickel and 44% titanium having a Martensitic state and an Austenitic state existing naturally on opposite sides of a narrow transition temperature range. The material undergoes a spontaneous transformation between states when the temperature changes across a transition temperature, which for Type 56 Nitinol has a transition temperature on the order of −30° C. In the Austenitic state, in which the material is used, it has very high strength and resists corrosion better than any other structural metal.

The connector

34

is made by boring or EDM cutting a cylindrical rod to produce a bore with an internal diameter about 6% less than the outside diameter of the barrel tube

36

, and cutting off a short section, perhaps 2″ long to form the connector

34

. The connector

34

is cooled below the transition temperature, conveniently by immersion in liquid nitrogen. Below the transition temperature, the material is in its Martensitic state and can easily be mechanically expanded in diameter, for example by a mechanical expanding mandrel of known construction. While still below the transition temperature, the connector

34

is slipped over the narrow end of the breech

32

, and the barrel tube

36

is inserted in the bore of the connector

34

. The connector

34

is now allowed to warm above its transition temperature, whereupon it spontaneously reverts to its memory size it had before being expanded when in the Martensitic state. It exerts a powerful radial force on the junction of the barrel tube and the breech, holding them together and reinforcing that region of the barrel

30

and the breech with prestressed Austenitic Nitinol having a yield strength of about 220 KSI. Conveniently, the mating ends of the barrel tube

36

and the breech

32

may be provided with shallow circumferential surface ridges on the outside surface to give the connector

34

a better grip.

Other forms of the first embodiment are shown in

FIGS. 2 and 3

wherein an integral barrel

50

and breech

52

for a rifle

54

, or an integral barrel

50

′ and breech

52

′ for a shotgun

56

are machined from a single blank of 60 Nitinol. The advantage of this design over that shown in

FIG. 1

is that the manufacturing and alignment of the barrel and breech are simplified. The disadvantage is that the amount of 60 Nitinol used to make the barrel increases substantially, and the amount of machining is increased. The machining of 60 Nitinol is difficult because it tends to use up the cutting tools quickly and cutting time is long. Another disadvantage is the absence of the coupling connector

34

which adds strength and toughness to an area of the gun where it is useful to have, but this shortcoming may be supplied if desired by a reinforcing band added for that purpose, or by a sleeve shown in the second embodiment described below.

The barrel blanks for the tube

36

and the integral barrel

50

/breech

52

may be made from a solid rod of Type 60 Nitinol, which is an intermetallic compound of 60% nickel and 40% titanium. It has a tensile strength of about 137-178 KSI and an ultimate yield strength of 222 KSI. Its elongation before rupture is only about 1%, but the strength is so great that it can safely contain the peak pressure of about 60 KSI of the burning propellant without significant elastic deformation, so the 1% elongation is never approached in a gun designed to withstand the peak pressures in the bore.

The coefficient of friction of conventional projectiles such as lead and gilding metal jacketed lead bullets with 60 Nitinol is substantially lower than it is with steel barrels, so the energy of the propellant is not wasted by conversion to heat in friction with the bore of the gun barrel. Instead, the projectile slips through the bore with little resistance, gaining.significantly greater muzzle velocity without heating the gun barrel as much as the same projectile would heat steel barrels.

The 60 Nitinol in the gun barrel

30

can be heat treated to a hardness of above 60 on the Rockwell C scale. It may be prudent to heat treat to a hardness somewhat less than 60+ to avoid brittleness, but a hardness even as low as the low to mid 40's on the Rockwell C scale would be an improvement over the soft barrels now in use. Erosion of the bore of a 60 Nitinol gun barrel would be significantly slower because of this greater hardness, and even more so because the coefficient of friction of most projectile materials in a 60 Nitinol bore is substantially lower than that in a steel bore.

The manufacturing process used to make the 60 Nitinol gun barrel are unlike those used to make conventional gun barrels because the 60 Nitinol material behaves differently than conventional steel barrel materials. The barrel blank is typically received from the supplier as a rough octagonal cross-section rod, since 60 Nitinol cannot be drawn or extruded but instead must be hot forged to the rod diameter from the cast ingot. The blank is centered in the chuck of a conventional barrel boring machine modified to slow the rotation and feed speed down to about 200-300 RPM and feed at a speed of about 0.25 inches/minute. Cutting is accomplished by rotating the barrel against the drill and flushing with copious quantities of cutting fluid, such as Cool Tool II made by Monroe Fluid Technology in Hilton, N.Y. It is important not to use too fast a rotation or feed speed to avoid raising the temperature of the working surface of the 60 Nitinol blank high enough to heat treat it, which would increase the hardness and make further cutting virtually impossible. The cutter is preferably a diamond bit, although tungsten carbide or titanium carbide coated cutters may be used with a slower feed rate.

When the bore is drilled completely through, the barrel may be left on the same chuck and turned to the desired outside diameter, which for a 0.223 barrel for an M-16 military rifle could be about ¾ inch in diameter, 75% smaller than the standard diameter barrel for that gun and about 45% lighter, but substantially stronger and stiffer. The barrel is turned at about 200 RPM against a diamond cutter to peal the barrel wall down to a uniform thickness to minimize any thermal distortion when the barrel gets hot due to uneven heating. The thickness will be about 0.25″-0.26″ which provides a barrel with a bursting strength in excess of that required in an M-16 rifle.

The coefficient of thermal expansion of 60 Nitinol is about 11×10

−6

/° C. The heat capacity of 60 Nitinol is higher than steel, on the order of 0.2 cal/g° C., and the thermal conductivity is low, only about 0.18 watt/cm

2

° C. so thermal effects, if any, develop slowly in the 60 Nitinol gun barrel. High temperature resistance of 60 Nitinol is excellent. Its yield strength remains constant to about 400° C., and then declines with the ultimate tensile strength gradually to above 700° C. so the surface material in the bore

42

retains its desirable properties noted above even when it becomes very hot.

An alternative technique for cutting the bore

42

through the 60 Nitinol blank is electrical discharge machining, using an apparatus shown schematically in

FIGS. 4 and 5

. Electrical discharge machining apparatus is made by several companies, including Mitsubishi EDM, MC Machinery Systems, Inc. and Hansvedt, Inc. commercially available from Perine Machine Tool Corporation in Portland, Oreg. in various forms. The EDM machines use a high power generator

60

that provides high frequency, high voltage, and high amperage electrically to a probe

62

to which it is connected by way of a conductor

64

. A barrel blank

66

is placed in a vessel

68

filled with coolant and is electrically connected to the generator

60

by a conductor

70

. The probe is preferably mounted vertically to avoid the bending effects of gravity that would act on a horizontally mounted probe. It is immersed in coolant that is pumped into the bore while the probe is advanced into the material. The high frequency current flows through the water from the conductor on the probe and into the workpiece where it burns away the material immediately adjacent the conductor, producing a clean, smooth precise cut. The probe

62

is advanced vertically into the vessel in axial alignment with the barrel blank

66

, and current from a conductor on the leading end

72

of the probe

62

flows into the adjacent surface of the barrel blank

66

, burning away material in the center to cut a clean and precise bore

74

.

There are two basic forms of EDM cutting: sinker EDM and wire EDM. The sinker EDM form uses a probe

62

having a cutting end

72

with a diameter about equal to the diameter of the bore

74

to be cut. The probe is preferably tubular in shape to cut a cylindrical plug out of the axis of the blank

66

instead of cutting away the entire mass of material from the bore

74

, and the cylindrical plug cut out to make the bore

74

can be used for other purposes. The other EDM cutting technique contemplated for use to make the gun barrel of this invention uses a thin sinker like the probe

62

, except that it is only about ⅛th inch in diameter to cut a small diameter hole axially through the barrel blank

66

adjacent to what will become the surface of the bore

74

. A wire is inserted through the hole and is held under tension at opposite ends of the hole by rollers that allow the wire to be advanced so it does not burn through while the cutting is proceeding. The generator

60

is energized and the wire is guided around a closed annular path by a guided support structure (not shown) to produce a cylindrical cut that is the inner wall of the bore

74

. An elongated cylindrical plug is cut loose by the annular wire cut and is removed and used for other purposes, such as a barrel blank for a smaller caliber gun barrel.

SECOND EMBODIMENT

Turning now to

FIG. 6

, a second embodiment of the invention is shown having a liner tube

80

made of 60 Nitinol and a reinforcing outer sleeve

82

around the liner tube

80

. As shown in

FIG. 6

, the liner tube

80

is integral with the breech

84

, but it could also be made separately as shown in FIG.

1

and attached to the breech with the sleeve

82

.

The reinforcing sleeve

82

provides additional strength and exerts a compressive preload on the 60 Nitinol liner tube

80

. Since 60 Nitinol has an elongation of less than 1% and tends to rupture when stressed beyond its yield point, a compressive preload exerted by the sleeve will provide additional strength to militate against the liner reaching the yield point and makes possible the use of a thin walled liner. In addition, use of a tough reinforcing sleeve

82

having an elongation greater than about 20% will provide additional protection against bursting of the barrel in the event that the yield strength of the liner

80

is exceeded, even with the compressive preload applied by the sleeve

82

, since the sleeve can yield without rupturing.

The reinforcing sleeve

82

is made of a nickel-titanium composition having two basic crystalline phases: a monoclinic Martensite state and an ordered body centered cubic Austenite state. These states are temperature dependent and undergo spontaneous enantiomorphic thermally induced allotropic phase transformations from one to the other and back again as the temperature of the material changes across a narrow transition temperature range. The sleeve

82

can be plastically deformed in its Martensitic state from an original shape to a deformed shape, and then will return to the original shape when warmed above its transition temperature. The sleeve

82

, if constrained against returning completely to its original form by the barrel liner

80

inside the sleeve, can exert a compressive force, up to its own yield strength, on the barrel liner

80

upon undergoing the phase change from Martensite to Austenite.

The material of the sleeve

82

is preferably a nickel-titanium intermetallic compound. Two types of sleeve material are contemplated by this invention: one form having a transition temperature colder than the lowest temperature which the gun is expected to experience in operation, for example, −30° C., and above −195° C., the boiling point of nitrogen, for a reason which is explained below. This first type would exist in its high strength Austenitic form during operation of the gun. One suitable material is 56 Nitinol, a binary intermetallic compound having 56% nickel and 44% titanium. Another suitable material is a ternary composition sold by Metaltex International Corp. in Albany, Oreg. under the name 220VC, and yet another one is a similar composition sold by Raychem Corp. in Menlo Park, Calif. under the name “Alloy A”.

A second form of sleeve material has a high transition temperature, higher than the operating temperature normally encountered in the use of the gun, so it would exist in its Martensitic state during normal operation of the gun. This second form is based on 55 Nitinol, which is a 50/50 atomic percentage intermetallic compound of nickel and titanium, with some doping materials added to raise the transition temperature, as known by those skilled in the metallurgy of nickel-titanium compounds.

The sleeve may be manufactured economically using a hot forming technique illustrated in

FIGS. 7-9

. A billet

90

of the sleeve material is positioned on an arbor

92

of a press over a clearance opening

94

. A hardened punch

96

of heat resistant material such as tool steel or Inconel is attached to the ram

98

of the press aligned over the clearance opening

94

. The punch is preferably sharpened or rounded at its distal end

100

to facilitate forming the hot material of the billet

90

. Depending on the aspect ratio of the billet

90

, it may be advisable to support the vertical sides of the billet

90

with retractable tooling

102

supported on guides

104

and biased against the billet by pneumatic actuators

106

or the like. Short thick billets will not normally require such support.

The billet

90

is heated to a high temperature, on the order of 1000° C., at which it becomes ductile and is positioned on the press arbor

92

. It may be brought back to ductile temperature with induction heating if it cools somewhat during transport from the furnace to the press, but the thermal conductivity of Nitinol is so low that the billet can normally be punched without supplemental heating. When positioned on the arbor

92

, the billet is pierced, as shown in

FIG. 8

, with a single rapid stroke of the press ram

98

which drives the punch

96

completely through the axial center of the billet

90

to form an axial bore

107

. The ram is quickly withdrawn and the pierced billet, shown in

FIG. 9

, is allowed to cool in preparation for the final processing.

When cooled, the pierced billet, now termed a sleeve blank

108

, is chucked onto a gun drill and the bore

107

formed by the punch

96

is cut to the desired diameter by a boring bar on which diamond cutters are mounted. A large power motor is required and a slow rotation speed for turning the sleeve blank and a slow feed speed are necessary for advancing the boring bar into the bore for cutting the bore to the required diameter to prevent the material from developing a strain hardened condition that makes further cutting difficult or impossible and damaging the cutters. Copious quantities of cutting fluid should be pumped through the bore

107

to carry away chips and remove heat.

One convenient manufacturing technique for expanding the sleeve

82

is to immerse the sleeve in a liquid nitrogen bath to cool it below the transition temperature for expanding in the Martensitic state. While in the liquid nitrogen bath, conveniently in a narrow vertical vessel, an expanding mandrel is inserted down into the sleeve and a rolling element is drawn through the mandrel, expanding it against the sleeve to increase the diameter of the sleeve between 4-8%, preferably about 6%. The rolling element is pulled through the mandrel, forcing it outward against the inside of the sleeve and forcing the sleeve to the selected larger diameter, plus springback. Then, while the sleeve is still in the liquid nitrogen bath, the liner is inserted down into the sleeve and the two elements are located at the exact desired position relative to each other. When positioned correctly, both elements are withdrawn together from the N

2

bath and allowed to warm to a temperature above the transition temperature of the sleeve material.

On passing through the transition temperature, the sleeve material reverts toward its memory shape and the sleeve

82

shrinks down onto the barrel liner

80

, exerting a radially compressive force on the liner

80

. The sleeve material exerts a force about equal to its tensile strength when it is constrained against returning to its original form by the liner

80

inside the sleeve

82

. The compressive force exerted by the sleeve creates a compressive stress in the liner that must be overcome by the pressure force inside the liner bore

74

by the projectile propellant gasses before the liner

80

is put into tension. Thus, the total pressure that the barrel liner preloaded in compression with the pretensioned sleeve can withstand is greatly increased over a plain 60 Nitinol barrel without the pretensioned sleeve.

Another form of sleeve is shown in

FIG. 12

as a series of stacked rings made of the same material as the sleeve

82

. These rings together form a sleeve when arranged contiguously along the barrel tube. Although the analysis for this type of sleeve is more complicated because of the stress discontinuities at the junctions of the sleeve rings, the ease of expanding the short sleeve rings in the cold Martensitic state and installing them on the barrel tube

80

offsets any such complication, especially since the rings can be made with thicker walls to counterbalance whatever effect the stress discontinuities might introduce.

THIRD EMBODIMENT

A third embodiment of the invention, shown in

FIG. 10

, includes a liner tube

110

made of 60 Nitinol and an outer sleeve

112

made of a low transition temperature nickel-titanium composition, such as 56 Nitinol, Alloy A, or 220VC described above, with an intermediate sleeve

114

between the liner

110

and the outer sleeve

112

. The intermediate sleeve

114

is made of a high transition temperature nickel-titanium composition having a high temperature Austenite state and a lower temperature Martensite state. The transition temperature of the composition selected for the intermediate sleeve

114

is preferably above the normal operating temperature of the gun barrel so that it remains in its Martensite state during normal operation. A suitable material for this application is 55 Nitinol, doped with gold, iridium, or other known dopants to raise the transition temperature. However, cobalt should not be used as a dopant since it adversely affects the properties of the 55 Nitinol.

The Nitinol intermediate sleeve

114

in its Martensitic state provides integral.damping of the barrel during firing to absorb shock and vibration and to prevent the barrel from developing natural frequency oscillations and whipping when it is fired. Such motions are inimical to accuracy of.the gun, especially in rapid firing situations, because they increase the uncertainty of the direction in which the barrel muzzle is pointed when the projectile leaves the muzzle. Nitinol in its Martensite state is an excellent damping material, having a specific damping capacity of about 40% when strained beyond 4%. Oscillations of a stiff structure, which otherwise would continue for minutes at a time, can be damped quickly, often within a small number of cycles, when a Martensitic damper is coupled to the structure and is strained cyclically with the structure while it oscillates.

The thickness of the Martensitic sleeve is selected to provide sufficient strain during firing of the gun to achieve the desired damping capacity. The material does not exhibit the damping capacity for small strain percentages, so a thin walled damping sleeve

114

is preferred, because a damping sleeve that is too thick may not be strained sufficiently to provide the desired high damping capacity.

A thin walled damping sleeve offers another advantage to the performance of the gun barrel, namely an increase in strength when subjected to large stresses. The sleeve is already stressed when it is installed, so not much additional stress is necessary to transform the material to the stress-induced Martensite state, wherein the strength increases from about 120 KSI to about 275 KSI or higher. Although the radial strain of the 60 Nitinol liner is not sufficient to strain the 55 Nitinol sleeve enough to transform it to stress-induced Martensite, the elongation it experiences during whipping or resonant vibrations of the barrel in the course of high repetition rate firing will often be sufficient to strain the 55 Nitinol sleeve material enough to transform it to the strain-induced state.

The gun barrel shown in

FIG. 10

has a separate breech

116

which could also be made of 60 Nitinol. The breech

116

has an outside diameter and an axial bore

118

with an inside diameter sized to receive a cartridge

48

or a powder canister (as used in naval guns) with a propellant charge for reacting in breech

116

to generate propellant gasses for propelling the projectile

46

from the gun.

The barrel liner

110

projects from the breech

116

in axial alignment therewith, with the axial bore

119

of the liner

110

aligned axially with the breech bore

118

for guiding the projectile propelled from the breech by the propellant gasses.

Making the breech

116

from a separate piece of 60 Nitinol saves material since it obviates the need to machine away a large amount of material around the barrel to reduce the outside diameter of the barrel to that shown in FIG.

10

. The breech

116

could also be conventional gun steel and is reinforced by the sleeves

112

and

114

, so it is capable of withstanding peak pressures of within the bore

118

of greater magnitude than a breech of comparable weight made of conventional breech materials and construction.

FOURTH EMBODIMENT

A fourth embodiment of the invention, illustrated in

FIG. 11

, includes a tube

120

made of a low transition temperature form of nickel-titanium composition. One such material is known as 56 Nitinol, and two other materials which would be suitable are the aforementioned compositions sold by Metaltex International Corp. under the name 220VC and Alloy A sold by Raychem. All of these compositions exist in a Martensitic state below a transition temperature and exist in an Austenitic state above the transition temperature. The transition temperature can be adjusted by the percentages of nickel and titanium, and also of the percentages of dopants, such as iron, aluminum, manganese, as well as other dopants mentioned herein and others known by those skilled in the art. These materials may be strain hardened through appropriate thermal processing, and exhibit unusually rapid work hardening. Barrel blanks of low transition temperature binary nickel-titanium compositions can be ordered from the supplier, such as Metaltex, with any desired transition temperature between −30° C. and −195° C., which would be suitable for making gun barrels according to this fourth embodiment of the invention, and there is no need to normalize the barrel with another heat treatment after cutting the bore

122

.

The barrel blank as received from the supplier is mounted on a gun drill and rotated against a diamond or tungsten carbide bit at low speed and low feed speed, for example, 0.75 inches/minute for a 0.223 bore. Attempting to cut the material at too high a rotation speed or too high a feed speed can result in work hardening that can increase the strength and hardness of the material to the point that it is nearly impossible to cut further. The bit must be kept sharp or the cutting speed will drop drastically and the energy input by the spindle will be converted to heat and the material be become virtually uncutable. Coolant/cutting fluid is flushed through the bore

122

at a high rate to flush out chips and prevent heat build-up which also can increase the difficulty of cutting. In addition, the bore

122

can be drilled using the EDM processes noted above for the 60 Nitinol tube, and also formed using the punch piercing technique noted for the sleeve manufacturing process discussed in connection with the second embodiment above.

The bore

122

can be drilled with a small diameter drill bit and then the bore enlarged with a boring bar using a titanium nitride coated cutting bit. The cutting speed must be kept slow: about 80-100 surface feet/second and a slow feed speed, on the order of about ½″/minute.

Rifling of the bore may be accomplished using the conventional broach cutting tools normally used for rifling, twisting as the broaching tool gradually as it is drawn back through the bore

122

to produce the desired pitch of the rifling. The cutting rate will be much slower for the nickel-titanium material than it is for convention steel barrels, but rifling of any desired depth can be produced with sufficient repetitions of the broaching operation.

A preferable technique for machining the rifling in the bore is electrochemical machining. An electrochemical probe, such as the one sold in a system made by Cacion, Inc. in Madison Heights, Mich., is inserted into the bore

122

filled with an electrolyte, and the system power supply is energized to produce a current flow from the probe to the bore. The current flowing in the electrolyte acts to remove metal adjacent the conductors on the probe. The cutting depth can be adjusted by the speed at which the probe is drawn through the bore, and the number of repetitions of moving the probe through the bore.

Low transition temperature intermetallic compounds of nickel-titanium in their Austenitic state and in stress induced Martensite have a yield strength of about 105-130 KSI and higher, and a hardness of about 35-42 on the Rockwell C scale. The material can undergo an elastic elongation of about 8% and a plastic elongation of as much as 60% before rupture. This extreme toughness makes the material extremely attractive for gun barrel material because of its propensity to yield and bulge when over pressured, rather than bursting in the face of the shooter. In its low temperature Martensitic state, these compositions have a lower yield strength, about 54 KSI, and hardness, about 25 Rockwell C, so it is easy to deform the sleeve to expand the sleeve diameter with an expanding mandrel or the like at low temperature, such as a liquid nitrogen bath, in the Martensitic state to prepare for the sleeve expansion step.

FIFTH EMBODIMENT

Two forms of the fifth embodiment of the invention, shown in

FIGS. 12 and 13

, include a tube

130

made of a low transition temperature nickel-titanium composition, such as the ones noted above for the fourth embodiment, used in the Austenitic state. The form shown in

FIG. 12

has an outer sleeve

132

made of rings

134

surrounding the liner tube

130

. The form shown in

FIG. 13

has an outer sleeve

132

that is a continuous sleeve

136

surrounding a barrel liner

138

, coupled by way of a coupling sleeve

34

to a separate breech as in the embodiment of FIG.

1

. The sleeves

132

, both in ring form and in continuous sleeve form, are formed of a nickel-titanium material and both exist in a state of tension when installed on the tubes

130

and

138

, exerting a compressive preload thereon, as discussed above in connection with the second embodiment. The material of the sleeves

132

could be either a low transition temperature nickel-titanium composition in the Austenitic state as noted for the second embodiment above, or it could be a nickel-titanium material having a high transition temperature, used primarily in the Martensitic state for compressively preloading the liner tubes

130

and

138

, and for damping. Although the tensile strength of the material in its Martensitic state is lower that when it is in its Austenitic state, it is sufficient to exert a tensile force of 20 KSI which can substantially preload the barrel liner in compression and add significantly to its burst strength.

The high specific damping capacity of nickel-titanium intermetallic compounds in the Martensitic state provide a benefit in additional to compressive preloading of the barrel liners

130

and

138

, namely, damping of whipping and resonant frequency vibrations of the barrel, especially during high speed firing. The sleeve

132

provides both of these functions when it is coupled with high interfacial pressure to the liner by shape memory contraction when is raised above the transition temperature after being expanded in the Martensitic state as discussed above.

A third form of the fifth embodiment, shown in

FIG. 14

, has a continuous sleeve

136

′ that extends the full length of a barrel liner

138

′ and connects the barrel liner

138

′ to a separate breech

32

using the same mechanism as the coupling sleeve

34

of

FIGS. 1 and 13

. The breech

32

could be a conventional steel breech whose strength is greatly reinforced with the compressive preloading of the sleeve, or could be made of the same material as that used for the barrel liner

138

′. The manufacturing of the sleeve

136

′ is simplified compared to the sleeves in the embodiment of

FIG. 10

because the outer diameter of the barrel liner matches the outer diameter of the breech

32

, so the sleeve

136

′ can be made as a perfectly uniform diameter cylindrical sleeve. It is made and installed on the aligned breech and barrel liner in the same manner as describe for the previous embodiments.

SIXTH EMBODIMENT

A sixth embodiment, shown in

FIG. 15

, has a barrel liner

140

and an outer sleeve

142

of low transition temperature intermetallic compounds of nickel-titanium in the Austenitic state, and an inner sleeve

144

integral with a breech

145

and made of a high transition temperature intermetallic compound of nickel-titanium in the Martensitic state. The outer sleeve

142

provides compressive preload for increasing the burst strength of the liner

140

, and also provides damping, as described in connection with the embodiment of FIG.

10

.

SEVENTH EMBODIMENT

Turning now to

FIG. 16

, a seventh embodiment of the invention is shown having a composite metal gun barrel

150

, including a steel outer tube

152

surrounding a Nitinol liner sleeve

154

through which an axial bore

156

extends. The outer tube

152

has a coupling structure

157

of known construction, shown schematically in

FIG. 16

, by which the barrel

150

is attached to the gun. The barrel

150

and coupling structure may be made of 4140 steel or other such material with a high Young's modulus. This embodiment is of particular value for long cantilevered gun barrels which tend to sag or droop at the distal end under their own weight, especially after extended periods of high rate of fire operation when the barrel gets hot, because of the improved stiffness provided by the steel outer tube

152

.

The liner sleeve

154

is preferably made of a low transition temperature Nitinol composition described above, such as the ternary compositions sold by Metaltex International Corporation in Albany, Oregon under the name 220VC, and a similar composition sold by Raychem Corp. in Menlo Park, Calif. under the name “Alloy A.” The binary intermetallic compound known as 56 Nitinol, having 56% nickel and 44% titanium can also be used. These materials are hard and tough, and all have shape memory characteristics, making them excellent candidates for gun barrel liner materials. Moreover, they have low thermal conductivity and heat up slowly in the presence of high temperature gasses to militate against heat flux into the barrel through the walls of the bore

156

, thereby delaying the overheating of the barrel during extended periods of use. The chemically inert and temperature resistant nature of Nitinol makes it tolerant of high temperature in the presence of corrosive influences that steel barrels would not tolerate. Of course, the 60 Nitinol liner tube

80

described above could also be used in place of the liner sleeve

154

in this seventh embodiment.

The embodiment shown in

FIG. 16

may be made in several ways, described below. The first method is by pressing the liner sleeve

154

into the steel outer tube

152

, as shown in FIG.

18

. The outer diameter of the liner sleeve

154

can be made slightly larger than the inner diameter of the outer tube

152

to create an interference fit when the sleeve

154

is pressed into the outer tube

152

. The interference fit prestresses the outer tube

152

in tension, thereby improving its resistance to drooping at the muzzle. The interference fit also prestresses the liner sleeve

154

in compression, thereby improving its bursting strength. The liner sleeve

154

is aligned with the outer tube

152

and is coated with a suitable lubricant such as graphite or boron nitride to reduce the sliding friction of the sleeve

154

in the tube

152

. A linearly guided hydraulically operated press head

158

presses the liner sleeve

154

straight into the outer steel tube

152

.

The liner sleeve

154

may be secured in the steel outer tube

152

by utilizing the shape memory effect of Nitinol. The Nitinol liner sleeve

154

is first immersed in a cryogenic bath, of liquid nitrogen for example, to reduce its temperature below the transition temperature so the Nitinol material transforms to its Martensitic state. In this state, the material is relatively soft and can be drawn to a longer shape with a smaller outer diameter. The drawing operation can done using either or both of the apparatus shown in

FIGS. 19 and 20

. In

FIG. 19

, a press head

160

driven by a hydraulic ram (not shown) drives the Nitinol liner sleeve

154

into and through an annular roller die

162

of known construction. The inside diameter of the die is smaller than the outer diameter of the Nitinol liner sleeve

154

and produces a pseudoplastic deformation of the liner sleeve

154

from its original shape. Alternatively, or in addition, the liner sleeve

154

may be gripped by a clamp

164

, shown in

FIG. 20

, and drawn through the die

162

by a puller mechanism

166

of known construction. The combination of both operations, that is, pushing the liner sleeve i

56

into the die

162

from one side and pulling from the other side offers the best combination of diameter reduction by longitudinal stretching under the pulling force exerted by the puller

166

and radial compression exerted by the die

162

.

The liner sleeve

154

must be in its Martensitic state during the drawing operation. The sleeve can be cooled in a liquid nitrogen bath and then quickly removed and mounted in the drawing apparatus for drawing to a smaller diameter. However, a preferred embodiment would be to draw the sleeve

154

while in the liquid nitrogen bath. This would require seals in the two ends of the tank holding the liquid nitrogen through which press head rod and the puller mechanism rod extend, and the tank would have to be twice as long as the liner sleeve

154

.

After drawing, the liner sleeve

154

could be removed from the tank and positioned inside the steel outer tube

152

or, preferably, could be positioned inside the steel outer tube

152

while still in the cryogenic bath tank, as shown in FIG.

21

. The assembled parts are removed from the tank and allowed to warm to room temperature. As the liner sleeve

154

passes through its transition temperature, it reverts back to is Austenitic state and spontaneously reverts to its original shorter, larger diameter shape, unless restrained. In this case, it is partially restrained by the bore of the steel outer tube which is sized accordingly. The Nitinol liner sleeve

154

exerts an outward radial force on the steel outer tube

152

, putting it into tensile preload. The outer tube

152

exerts a radially inward compressive force on the liner sleeve, putting it into compressive preload. The preload stress in the liner sleeve

154

and the outer tube

152

improves the stiffness of the composite metal barrel to resist drooping at the muzzle end, and also improves the burst strength of the barrel

150

.

Another form of the seventh embodiment is shown in

FIGS. 22-26

wherein the liner tube

154

is sized to slide with a snug fit into the steel outer tube

165

. Although this form of the gun barrel does not benefit from compressive prestressing of the liner sleeve

154

or tensile prestressing of the steel outer tube

165

, it has value in simplifying the manufacturing for purposes of testing, wherein liner sleeves

154

of various types and calibers can be tested in a single gun. It would also be of interest in sport guns wherein drooping of the gun barrel at the muzzle end is not a factor and where interchangeable barrels would be a desirable feature.

A substantial frictional force is exerted by the projectile on the liner sleeve

154

when the projectile travels toward the muzzle, and in the embodiments of

FIGS. 22-26

, wherein the liner sleeve

154

is not fixed in the outer tube by an interference fit or the like, this force is reacted by the outer tube to prevent the projectile from taking the liner sleeve

154

with it when the gun is fired. This reaction force for retaining the liner sleeve

154

in the steel outer tube may be provided by end caps at the muzzle end of the outer tube. Two different end caps

166

and

168

are shown in place in

FIGS. 23 and 25

. The end cap

166

, shown in

FIGS. 22-24

, is a steel nipple having a center bore

170

larger than the bore

156

through the liner sleeve

154

. Suitable spanner recesses are provided in the front end of the end cap

166

to facilitate threading the end cap into an internally threaded end portion

172

on the muzzle end of the outer tube

165

. An internally projecting radial flange

174

at the breech end of the outer tube

165

traps the breech end of the liner sleeve

154

in the outer tube

165

so the barrel can be handled as a unit. Another form of outer tube

176

shown in

FIG. 24

has an outwardly projecting radial flange

178

at its breech end by which the barrel may be attached to the gun. In this configuration, a gland nut (not shown) of know construction captures the flange

178

and clamps it to the breech of the gun, trapping the breech end of the liner sleeve

154

against the breech.

Another configuration of an end cap for retaining the sleeve liner

154

in the outer tube is shown at

168

in

FIGS. 25 and 26

. The end cap

168

is internally threaded and engages external threads

178

at the muzzle end of the outer tubes

180

and

182

. An inwardly projecting radial flange

184

engages the muzzle end of the liner sleeve

154

to trap the liner sleeve in the outer tube against axial translation relative thereto when a projectile is fired from the gun barrel.

Another form of the seventh embodiment, shown in

FIGS. 27-29

, uses a compression clamp

190

at one end of the gun barrel to secure the liner sleeve to the steel outer tube

191

. The liner sleeve, shown at

192

in

FIGS. 27 and 28

, has a shallow annular groove or cannelure

194

adjacent its muzzle or breech end. The width of the cannelure is equal to the width of the compression clamp

190

which extends into the cannelure

194

with a snug fit. The clamp

190

is made in two identical diametrical halves

196

a

and

196

b

which are fastened together by two machine screws, such as Allen head screws

198

, extending through a shouldered hole in one clamp half and threaded into a threaded hole

202

in the other clamp half, as shown in

FIG. 29. A

series of bolts or machine screws

204

may be provided to attach the clamp

190

to the steel outer tube

191

, especially if the clamp is near the breech end of the gun. Conveniently, the clamp

190

can be incorporated into the coupling structure

157

by which the barrel is attached to the gun.

The discussion above mentions small arm caliber hunting and military weapons, but the invention is also expressly intended for use in larger caliber weapons such as 0.50 caliber machine guns, 20 and 30 millimeter cannons, high firing repetition rate cannons in particular, and in field artillery, mortars, rocket launchers, and naval guns. It would also find application in ultra-high velocity guns such as rail guns and in large caliber, high rate-of-fire liquid propellant guns. The benefits of the invention may be more important to high muzzle velocity and larger caliber weapons than to the smaller caliber weapons because the problem solved by the invention have more serious consequences in big guns, high rate-of-fire guns, and high muzzle velocity guns than in smaller individual weapons.

Artillery and large naval gun barrels are expensive, in part because of the high fabrication cost of making the large monolithic forging which forms the barrel blank, and because of the cost of turning and boring the blank. An embodiment of the invention, shown in

FIG. 30

, is a gun barrel

208

having an inner liner

210

surrounded by an outer tube

212

. The inner liner is made of a plurality of segments

214

, shown separately in

FIG. 32

, having concentric inner and outer cylindrical surfaces

216

and

218

, and having radial side surfaces

220

on planes that intersect on a line

222

at the center of curvature of the cylindrical surfaces

216

and

218

, that is, on the axis of the barrel bore. The number of segments

214

in a barrel will vary depending on the thickness of the segment

214

and the diameter of the barrel, but 4-6 segments will usually suffice. It is preferable to use an aspect ratio, that is, segment thickness divided by radius of curvature of the outer cylindrical surface

218

that is large enough to withstand the buckling forces exerted by the projectile passing through the bore, and the twisting forces exerted by the projectile on the rifling ridges. The thickness of the outer tube

212

needs to be sufficient to withstand the hoop stress created by the cup pressures of the burning propellant behind the projectile, and also to support the barrel against sagging under the influence of gravity.

The barrel is assembled by producing the segments

214

, as described below, and assembling them on a mandrel

224

, as shown in FIG.

31

. The mandrel is preferably a two piece construction with a helical outer member hating a cylindrical outer surface and a tapered inner surface, and an inner tapered member that can be inserted into the helical outer member to provide radial support for the segments

214

but can be withdrawn to allow the helical outer member to retract radially so it can be pulled out of the bore after assembly of the barrel components. A tool of this general construction is known as a lap and is used for precision honing of holes. The scale of the lap used as a mandrel in this application would be much bigger than normal laps.

The assembled segments on the mandrel

224

are immersed in liquid nitrogen or otherwise cooled, while the outer tube

212

is heated to an elevated temperature of above 300° C.-400° C. The outer tube

212

and the mandrel/segment assembly are quickly telescoped together, preferably on a guide apparatus that facilitates rapid and precise telescoping movement of the components together. The heat transfer from the outer tube

212

to the segments

214

causes a rapid temperature equalization, which contracts the outer tube

212

and expands the segments

214

into intimate and high pressure contact. After temperature equalization, the inner tapered member of the mandrel is dislodged and the helical outer member is pulled out, leaving the segments jammed together in place in a state of compression. The resulting barrel

208

has an outer tube

212

that is prestressed in tension, and a hard, slippery and corrosion resistant liner sleeve

210

prestressed in compression. If desired, the bore through the liner sleeve

210

can be reamed and rifled using conventional tools made for those functions.

The outer tube is preferably a steel alloy such as

416

stainless steel or 4140 gun steel with a high Young's modulous and a high coefficient of thermal expansion. It is formed in the tube shape by conventional gun drilling or by rolling a plate of material and welding along the facing edges, and then reaming the tube to produce an accurate cylindrical bore. The segments may be machined from a forged ingot of low transition temperature Nitinol, such as the 220VC described previously, or from an ingot of forged Type 60 Nitinol. The 220VC material machines well by conventional machining processes, so no special procedures are needed. The Type 60 Nitinol is much more difficult to machine and is best cut with polycrystaline cubic boron nitride (PCBN) cutters powered with high horsepower motors at high cutter surface speeds and low feed rates and shallow depths of cut.

A preferred method of making the segments

214

, illustrated schematically in

FIGS. 33

,

34

and

32

, starts with a flat rolled slab or plate of Nitinol which is cut into elongated liner pieces

226

having opposite edges

220

disposed at an angle which will lie on radial planes intersecting at the bore axis after the slab

226

is formed into a cylindrical segment, as shown in FIG.

32

. Alternatively, the cutting or grinding operation for the edges

220

could be postponed until after the segments are formed into the cylindrical shape. The cutting can be done with abrasive water jet or wire EDM. However, the preferred cutting technique is laser cutting with a high power laser and a jet of gas such as nitrogen or argon to blow the molten metal out of the kerf. The laser makes a very clean cut and is much faster than water jet or wire EDM, however the current state of development of laser cutting apparatus limits the depth of cut, so thick slabs may have to be cut with the other techniques. Conventional cutting techniques may be used for the 220VC type Nitinol since it is easier to machine.

The liner pieces

226

are formed as illustrated in

FIG. 34

by heating them to an elevated temperature between 600° C. and 950° C., preferably about 800° C., and pressing them into a die

228

having a die cavity with a cylindrical forming surface

230

. The. radius of curvature of the die cavity cylindrical forming surface

230

is equal to the desired outside radius of curvature of the segment

214

. The pressing of the the liner pieces

226

into the die cavity

230

is done preferably with a matched male die (not shown) having a cylindrical die surface with a radius of curvature about equal to the radius of the bore of the barrel

208

. The segment is held in the die until it cools to a cool temperature below 300° C., preferably about 200° C. and is then removed from the die

228

.

The faying surfaces of the segment edges

220

in the assembled barrel liner

210

should match closely without gaps, so they are preferably ground to provide exactly matching surfaces in the assemble barrel

208

. The grinding may be done with a CNC grinding apparatus using a PCBN grinding wheel or belt. The depth of cut should be relatively shallow, on the order of 0.001″-0.003″ and the feed speed should be slower than conventional grinding.

Obviously, numerous modification and variations of the described preferred embodiments will occur to those skilled in the art in light of this specification. For example, new formulations of nickel-titanium compositions will continue to be developed and these compositions may be logical candidates for use in this invention. Also, many function and advantages are described for the preferred embodiments, but in some uses of the invention, not all of these functions and advantages would be needed. Therefore, we contemplate the use of the invention using fewer than the complete set of noted functions and advantages. Moreover,numerous species and embodiments of the invention are disclosed herein, but not all are specifically claimed, although all are covered by generic claims. Nevertheless, it is my intention that each and every one of these species and embodiments, and the equivalents thereof, be encompassed and protected within the scope of the following claims, and no dedication to the public is intended by virtue of the lack of claims specific to any individual species. Accordingly, it is expressly to be understood that all the disclosed species and embodiments, and the numerous modifications and variations, and all the equivalents thereof, are to be encompassed within the spirit and scope of the invention as defined in the following claims, wherein I claim: