Add SCP-127 and heres why
The living gun will be like any other gun except that ammo cannot be instantly reloaded instead the gun creates more ammunition slowly over time. the pro to this is that you can never run out of ammo the con is that if you need more you will have to wait for more ammunition to be created.
The gun should be contained in a room that requires some form of keycard access. also if you are able to kill MTF in the game and there are actual guns, the living gun should be less effective per shot than the real ones.