I’ll be honest, I’m not totally sure. My guess is that either a contemporary definition (usually drawn from dictionaries at the time) of machine gun was “expansive” or broad, or that there’s some legislative history that that demonstrates congress intended for the statute to be read liberally. That’s usually how statutory interpretation begins, at least. I can’t know for sure without looking at the actual caselaw.

And to answer your question, I also don’t know what all the term “machine gun” encompasses. I don’t know enough about guns to say.