Colt has made a beautiful reproduction of the 1887 Bulldog Gatling gun, which fires a .45-70 cartridge as fast as you can turn the crank. As a legal issue, a gatling gun is not NFA, because it does not fit the definition of a machine gun. However, if you had a drill that fit the shaft, and put that on the gun, you’d be guilty of a violation of 922(o), and you’d go to jail. I’ve always wondered if they could get someone on constructive possession on that count, if they had a drill that would fit the shaft, even if they never used it as such, on the grounds that they had an easy conversion. Remember that constructive possession is whatever a US attorney can convince a jury it is.