An electronic button that operates multiple triggers is considered "the trigger" - think drill that causes a trigger to be depressed multiple times; the drill is the machine gun - ATF has already addressed this, before paintball was popular.
Devices that permit a gun to fire on the press and on the release of a trigger were invented many years ago - google "staple Mini-14" or read this:
http://www.thehighroad.org/archive/i...p/t-62411.html
People have tried to come up with many ways to get around what does and does not constitute a machine gun. The bottom line is that most are illegal or simply don't work. Every now and then something actually works. The only three things that I know of that simulate full-auto fire that are legal are: Bump-fire stock; gattling gun set-ups; "staple" thingy on a mini-14.