Fostech Outdoors are about to ship the DefendAR-15 bump fire stock for the AR-15. It is selling for $500.

It works just like the other bump fire stocks on the market: the rifle sits in the stock but is free to move backwards and forwards, the operator then pushes the rifle forward forcing the trigger against their trigger finger, the recoil of the fired round pushes the rifles back reseting the trigger, the operator’s continued forward pressure restarts the cycle. Repeat until out of ammunition.

[ Many thanks to jdun1911 for emailing me the link. ]