I agree with #2 above and add, a machine gun can have any barrel length, pistol grip, stock etc. There is no such thing as a SBR machine gun. In the eyes of the NFA branch a muffler or silencer is in itself a firearm and requires it’s own stamp all of the time. A “short barrel” never needs a stamp (without getting into the “intent” part) just a stamped receiver (MG or SBR) to go onto unless it’s a pistol receiver.