1. A firearm adapter, comprising:

an upper plate member on the adapter having an upper side and a lower side; a clamp on the upper side of the adapter member, the clamp having clamp edges for allowing the adapter to be clamped to picatinny rails located beneath a firearm, the upper plate member having an indention in the lower side, the indentation having a first end wall and a second end wall, which is opposite to the first end wall; a swing plate pivotally attached to the first end wall of the indentation , the swing plate having a lower surface with picatinny side edges for supporting an accessory thereon; and a sliding switch on the adapter for allowing the swing plate to be released from a horizontal locked position to be able to rotate to a substantially vertical position.

2. The firearm adapter of claim 1 , wherein the sliding switch includes: an angled raised surface for allowing a finger of a user to push against.

3. The firearm adapter of claim 1, wherein the sliding switch includes:

a spring for biasing the sliding switch to the locked position.

4. The firearm adapter of claim 3, further comprising: a set screw for adjusting the biasing extension of the spring.

5. The firearm adapter of claim 1 , further comprising : a catch on a free end of the swinging plate for catching onto a protruding end on the switch, so that the swinging plate is held in the locked position.

6. The firearm of claim 1 , further comprising:

a spring loaded latch for locking the swinging plate in the substantially vertical position.

7. The firearm adapter of claim 1 , further comprising:

a first spring for biasing the sliding switch to the locked horizontal position; and a second spring for locking the swinging plate to the substantially vertical position.

8. A method of attaching a foidable accessory mounting plate to a firearm, comprising the steps of:

providing a firearm having opposite facing picatinny rails underneath the firearm;

providing a top plate member having an upper side with clamp edges and a lower side with an indentation;

providing a bottom plate member that is sized to substantially fit within the indentation on the lower side of the top plate member;

clamping the clamp edges of the top plate member about the picatinny rails;

pivotally attaching one end of the bottom plate member adjacent to a wall within the indentation in the lower side of the top plate member;

locking the bottom plate member into a folded horizontal position parallel to the top plale member by a sliding switch being moved in one direction; and

releasing the bottom plate member to rotate to a substantially vertical position by moving the sliding switch in an opposite direction.

9.. The method of claim 8, further comprising the step of:

spring biasing the sliding switch toward the one position.

10. The method of claim 8, further comprising the step of:

locking the bottom plate member to the substantially vertical position by a spring.

11. The method of claim 8, further comprising the steps of:

biasing the sliding switch toward the one position by a first spring; and

locking the bottom plate member to the substantially vertical position by a second spring.

12. A folding rail adapter for firearms, comprising:

a first plate shaped member having an upper side and a lower side, a first end, and a second end opposite to the first end, the upper side having clamp edges for ciampmg about picatinny rails located underneath a firearm, the first plate shaped member having an indentation in the lower side;

a second plate shaped member having an upper side and a lower side, a first end and a second end opposite to the first end, the lower side having a first longitudinal picatinny rail a and a parallel second longitudinal picatinny rail located between the first end and the second end of the second plate shaped member; and

a hinge for allowing a first end of the second plate shaped member to pivot in and out of the indentation in the lower side of the first plate shaped member; and

a raised switch for allowing the second plate shaped member to pivot from a horizontal position with the first plate shaped member to a substantially vertical position.

13. The folding rail adapter of claim 12, wherein the raised switch includes :

a moveable latch on the adapter having a lock position for locking the second plate shaped member in the horizontal position, and a release position for allowing the second plate shaped member to move to the substantially vertical position .

14. The folding rail adapter of claim 12, wherein the raised switch includes :

a slidable switch on the adapter having a lock position for locking the second plate shaped member in the horizontal position and a release position for allowing the second plate shaped member to move to the substantially vertical position .

15. A method of substituting folding picatinny rails for fixed picatinny rails mounted to firearms, comprising the steps of:

providing a firearm with an existing mounted plate having pair of fixed parallel picatinny rails, the existing mounted plate having a generally planar plate configuration with a set of mounting holes located therethrough for allowing fasteners to mount the existing mounted plate to a rail location on the firearm ;

providing another plate having a pair of folding parallel picatinny rails, the another plate having a generally planar plate configuration with another set of mounting holes therethrough that are located at identical locations to the mounting holes in the existing mounted plate;

removing the existing pair of parallel picatinny rails from the firearm; and

substimting and mounting the another plate with the pair of folding rails to the rail location on the firearm, by using the another set of the mounting holes with the fasteners .

16. The method of claim 15, further comprising the step of:

hinging the pair of folding parallel picatinny rails to the another plate, so that the pair of folding parallel picatinny rails pivots from a horizontal position with the another plate to a substantially vertical position to the another plate.

17. The method of claim 16, further comprising the steps of:

latching the folding parallel picatinny rails to the horizontal position; and

unlatching the folding parallel picatinny rails to be allowed to pivot to the substantially vertical position.

18. The method of claim 17, wherein the latching step includes the step of:

rotating a latch to lock the folding parallel rails to the horizontal position; and

rotating the latch to unlock the folding parallel rails to be allowed to pivot to the substantially vertical position.

19. The method of claim 15, further comprising the step of:

providing the another plate with a parallel set of fixed picatinny rails that are not foldable, the another plate of fixed parallel fixed picatinny rails being aligned end to end with the folding parallel picatinny rails.

20. The method of claim 15, further comprising the steps of:

providing the another plate with a first parallel set of fixed picatinny rails that are not foldable, the first parallel fixed picatinny rails being aligned to one end of the folding parallel picatinny rails; and providing the another plate with a second parallel set of fixed picatinny rails that are not foldable, the second parallel fixed picatinny rails being aligned to an opposite end of the folding parallel picatinny rails.

21. The method of claim 20, further comprising the step of:

providing a base member that is fixed to both the first parallel fixed picatinny rails and to the second parallel fixed picatinny rails.

22. The method of claim 15, further comprising the steps of:

providing an upper flat surface on the another plate for mounting against the firearm; and providing the folding rails along edges of a lower surface of the another plate.

23. A folding picatinny rail adapter for being attached to existing mounting holes used for fixed picatinny rails located on firearms, comprising:

a plate having a pair of folding parallel picatinny rails along opposite side edges, the plate having a generally planar plate configuration; and

a set of mounting holes through the plate that are located at identical locations to mounting holes on firearms used for attaching fixed picatinny rails to the firearms, wherein the folding picatinny rail adapter are adaptable to be substituted for existing parallel fixed picatinny rails on the firearm.

24. The folding rail adapter of claim 23, further comprising:

a hinge for allowing the folding parallel picatinny rails to pivot from a horizontal position with the another plate to a substantially vertical position to the another plate.

25. The folding rail adapter of claim 23, further comprising:

a latch for latching the folding parallel picatinny rails to the horizontal position; and for unlatching the folding parallel picatinny rails to be allowed to pivot to the substantially vertical position.

26. The folding rail adapter of claim 25, wherein the latch includes:

a rotatable latch that is rotated in one direction to lock the folding parallel rails to the horizontal position, and is rotated again to unlock the folding parallel rails to be allowed to pivot to the substantially vertical position.

27. The folding rail adapter of claim 23, further comprising:

another plate with a parallel set of fixed picatinny rails that are not foldable, the another plate of parallel fixed picatinny rails being aligned end to end with the plate having the folding parallel picatinny rails.

28. The folding rail adapter of claim 23, further comprising:

an first additional plate with a first parallel set of fixed picatinny rails that are not foldable, the first parallel fixed picatinny rails being aligned to one end of the folding parallel picatinny rails; and

a second additional plate with a second parallel set of fixed picatinny rails that are not foldable, the second parallel fixed picatinny rails being aligned to an opposite end of the folding parallel picatinny rails.

29. The folding rail adapter of claim 23, further comprising:

a connecting base member that is fixed to both the first additional plate and to the second additional plate.

30- The folding rail adapter of claim 23 , further comprising:

an upper flat surface on the plate for mounting against the firearm; and

a lower surface on the plate having the folding rails along edges of the lower surface.

31. A folding picatinny rail adapter for being attached to existing mounting holes used for fixed picatinny rails located on firearms, consisting of:

a plate having a generally planar plate configuration, the plate having a base with an upper flat surface for abutting against a surface of a firearm, and the plate having a lower surface;

a parallel pair of picatinny rails on opposite sides of the lower surface of the plate, and the rails having a first end and a second end;

a hinge for pivotally attaching the first end of the picatinny rails to the lower surface of the planar plate;

. a set of mounting holes through the plate that are located at identical locations to mounting holes on firearms used for attaching fixed picatinny rails to the firearms; and

a latch for latching the folding parallel picatinny rails to a horizontal position along the lower surface of the plate and for unlatching the folding parallel picatinny rails to be allowed to pivot to a substantially vertical position to the plate.

32. The folding rail adapter of claim 31 , wherein the plate includes:

a parallel set of fixed picatinny rails that are not foldable, the parallel fixed picatinny rails being aligned end to an end of the folding parallel picatinny rails.

33. The folding rail adapter of claim 31 , wherein the plate includes:

a first parallel set of fixed picatinny rails that are not foldable. the first parallel fixed picatinny rails being aligned to the first end of the folding parallel picatinny rails; and

a second parallel set of fixed picatinny rails that are not foldable, the second parallel fixed picatinny rails being aligned to the second end of the folding parallel picatinny rails.

34. The folding rail adapter of claim 31 , wherein the base includes:

a connecting member that is fixed to both the first parallel set of fixed picatinny rails and to the second parallel set of fixed picatinny rails.