Update, the Final Rule is supposed to be published in the Federal Register on January 31st last I read; and
@ajole , the Heller case says 2A
also protects arms that didn't exist at the time of 2A writing, the same way 1A protects internet, cell phone comms, and tv....

as to the "common use" thing;
Caetano V. Massachusetts went to SCOTUS
but was unanimously
vacated and remanded and the
opinion of Honorable Judge Alito, with Honorable Judge Thomas concurring; is that 200,000 stun guns makes them
not "Dangerous and Unusual" weapons, and that because they didn't exist at the time of 2A's writing, doesn't mean that they are outside 2A Protection, repeating from Heller. The State of Massachusetts chose to
drop all charges... and in
Ramirez v. Commonwealth, it was found that stun guns are protected under 2A and the absolute Prohibition of stun stuns violated 2A...but as it didn't go all the way to SCOTUS, and as SCOTUS didn't make a decision, it is not "precedent"

but... there's a possibility that the
entire NFA could be binned to the trash of history through the "common lawful use vs "dangerous and unusual " weapons route.