"bump firing" can be done with many semi-auto weapons and allows multiple shots for one pull of the trigger. Bump firing uses the rifle's recoil, forward pressure on the forearm, plus a firm fixed finger on the trigger and provides a rate of fire similar to full auto. Since the gun is not modified in any way, bump firing is legal in most states.

Now some folks have come up with a special stock that makes bump firing super easy to do, controllable, and safe. No more hooking your finger around your belt loop and firing from the hip!

it costs $320 so if you have an AR, 3 bills burning a hole in your pocket, and want nothing more then to bust caps ala' full auto, i'd act now before they make them illegal.