The crankfire units work very well on the 1919A4 semi guns, I have one and it's pretty neat.
The wording on NFA weapons requiring registration is whether the weapon will fire more than one shot with a single function of the trigger. By definition, a Gatling gun fires only one time with a single function of the trigger (crank). This also applies to the Browning machine gun crank fire units. If I turn the crank very slowly, I can fire only one single shot at a time. A "real" machine gun, however, will fire once the trigger is pulled, and continue to fire, uintil it is out of ammunition, or the trigger is released.