Another key fact to remember in regards to the NFA, it was passed after Prohibition had been repealed.
I've often read that it was passed merely to give a bunch of soon to be out of work Treasury Agents something to do.
Also, from my readings, most of the Thompsons and BARs used by the bad guys during the "Roaring '20s" were stolen from police stations and National Guard armories.
NRA Life Member
"Boy Scout Problem #18: Living with the fear that one day someone will find out you can't tie anything more complicated than a square knot."