If you see in the
video of the RS prototype it seems that gravity is what "pulls" the stock out and locks it perhaps just a bit of weight in the end of the stock.
also as to not take apart or cut apart the gun maybe a gun with a railed frame would work well it could just slide in ad attach like a pistol scope mount the one for my glock slides on the lower rail has 2 set screws that push up on the lower part of the frame where the notch for the rail is and then the trigger pin thing is replaced by a nut and bolt like this
mount the second set of images