I was reading an ATFE approval letter for one of those rigs that mounts 2 ruger 10/22s on a tripod when an idea suddenly occured to me. This particular rig uses a hand crank that turns a couple cams. The cams rock a bar back and forth, firing each rifle in turn rapidly. The letter stated that as long as it was hand cranked the setup was semi-auto. It further stated that adding a motor to the setup would make it an illegal full auto. The reason given was the switch for the motor then became the trigger and fed law states that a machine gun is: "Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger."
Many other hand cranked rapid fire devices exist and most of them have an approval letter stating the same thing. Add a motor, switch becomes trigger, device becomes a machine gun. The key words here are "single function of the trigger". Say one connected a stepper motor to a trigger actuater on a semi automatic firearm. A small built in circuit would limit the action so that one pull of the trigger switch equals a single step of the motor and a single round fired. Pull 3 times slowly, 3 shots fire. Now, this circuit could also have a rapid fire mode. Pull the trigger twice in say a half second or so (think double mouse click) and the circuit continues to fire the gun until the trigger switch is released. Or let the circuit "store up" the pulls. If you rapidly pull the trigger it counts how many pulls were made then waits to activate the motor until the pulls stop. Say you rapidly pulled the trigger 20 times then stopped. A second later 20 shots are fired in about a half second. 20 shots, 20 pulls. Neither situation involves more than one shot for a single function of the trigger.
Now, I'm sure ATFE is gonna come up with some way of saying this is illegal, the timer circuit itself becomes the trigger, etc. But it would be an interesting project for an SOT to play with, who knows it just might pass.