What is claimed is:1. A training device for a combat vehicle with a heavy weapon, comprising:a) a replacement practice barrel which is shorter than a regular weapon barrel and has attachments at an inner end for fastening same to a combat vehicle;b) a magazine at an outer end of the practice barrel for accommodating rammed practice shells and having magazine shafts offside an axis of the barrel and into which decelerated rammed practice shells are forwarded and a brake accommodated therein;c) wherein the practice shells have essentially the same weight and dimensions as regular shells and an impact absorbing portion for accepting the impact of the brake; andd) an accessory device for removing the practice shells from the magazine comprising a framework that travels on rollers and is provided with a platform for operating personnel and with a hydraulic lift that accepts used practice shells from the magazine and lowers them to the ground.2. The training device as in claim 1, wherein the barrel has a smooth bore and is mounted in a cradle in the vehicle and secured with a breech exactly like a regular barrel.3. The training device as in claim 1 or 2, wherein the magazine is provided with an accommodation attached concentric to the barrel and in or on the front of which is mounted the brake and on each side of which is mounted the magazine shafts that can accept several practice shells provided by a conveyance.4. The training device as in claim 3, wherein the magazine shafts mounted on each side of the accommodation slope at a slight angle to a plane defined by the axis of barrel and an axis of elevation.5. The training device as in claim 1, further comprising an extraction opening on an outer end of each magazine shaft and caps for closing same, each connected to an outward-extending arm connected to a handle.6. The training device as in claim 5, wherein each arm can be locked in place by locking mechanisms while the caps are closed.7. The training device as in claim 3, further comprising a rail that supports the shells in the accommodation as they enter and are decelerated, and wherein the rail drops against the force exerted by springs, and is bent such that, once it has dropped, the decelerated shell will roll off of it into, initially, a first magazine shaft.8. The training device as in claim 7, wherein the first magazine shaft is just wide enough that, once it has accepted a prescribed maximum of shells, the next shell to enter the accommodation will be intermediately displaced with respect to a second magazine shaft and will drop into it.9. The training device as in claim 7 or 8, wherein the rail is articulated to free ends of two suspension levers, each of which is articulated to a front and rear of the magazine in the vicinity of the first magazine shaft and tending to swing down against the force exerted by tension springs.10. The training device as in claim 3, further comprising a latch positioned at an entry into the accommodation to prevent a rammed shell from sliding backward.11. The training device as in claim 1, further comprising releasable latches in the magazine shafts that determine the position of the practice shells in the shafts.12. Training device as in claim 3, wherein the brake has a shock absorber that a tray slides back and forth on along the accommodation, wherein the tray has a stop on the side facing an incoming shell and resting against the shell.13. The training device as in claim 5, wherein settings of the arms control the shells in the magazine.14. The training device as in claim 13, further comprising electronic controls under a hood on top of the magazine.15. The training device as in one claim 1, wherein the shell has an ogive at the front portion and a removable base and wherein the impact absorbing portion comprises an impact accommodating shoulder turned out of a section adjacent to the ogive.16. The training device as in claim 15, wherein the ogive is attached to the shell by a threaded joint and wherein the shoulder is in the vicinity thereof.17. The training device as in claim 16, further comprising a replaceable plastic shock-absorption ring around the impact accommodating shoulder.18. The training device as in claim 15, wherein the shell further comprises a screw-in fuse.19. The training device as in claim 1, wherein the accessory device comprises four uprights against the framework, along which the elevator with the tray travels and wherein the hydraulic lift has hydraulic components that raise and lower the elevator and are accommodated in a bottom of the framework.20. The training device as in claim 19, wherein the tray has a sloping bottom and a rocker that can accommodate the shells and which is mounted on shock absorbers at a rear portion where the shells drop in.21. The training device as in claim 19 or 20, further comprising adjustment-and-securing structures mounted on the shafts and thrust into slots in forward uprights.