Another key fact to remember in regards to the NFA, it was passed after Prohibition had been repealed.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
Me: "The .45 GAP was an answer to the question nobody asked."
Trooper pal: "Says the guy who thinks .38 Super and 10mm are viable pistol calibers and gets mad when Wal-Mart doesn't have them."