Someone already makes that in a crank device you affix to the trigger guard,,,
It's called the BMF activator (Click here please),,,
Lotsa fun for $19.95.
I know, but my concept is a geared pump device (think shotgun pump) that mounts under a rifle that does the same thing as a the BMF activator....just better. With each stroke of the pump - forward and backward - it would cause a piston to press the trigger at a rate of up to 4-5 times per pull/push of the pump. It would permit you to hold, point and aim the gun more normally than would a hand crank, yet still simulate full-auto fire.