The major target of the NFA was definitely machine guns following crime during Prohibition, the highly publicized St. Valetine's Day Massacre and the years of activity from Depression era gangsters. Donks didn't have a clear majority of both houses and the Presidency until 1933.The legislative history of the NFA makes it clear that the intent was to ban through taxation. The inflation-adjusted $200 tax from 1934 would be roughly $4,400 today for every SBS, SBR, and silencer. Same for automatics, except the 1986 Hughes Amendment to FOPA already drove the price through the roof by banning (not taxing, but actually banning) all new machine guns for the civilian market.
That was the first ever actual gun ban under federal law, so it doesn't survive "history and tradition" under Bruen. That will be a law suit for a different day, but I'm sure there are several litigants lined up and waiting.
As has been pointed out, NFA'34 started out as a handgun ban (through taxation, because they knew they couldn't actually ban guns). SBS/SBR were added only because it's relatively easy to cut down a rifle/shotgun to make it "concealable" like a handgun... but handguns were dropped from the law because that was politically untenable.
For reasons of political insanity, SBR/SBS stayed in the law for no logical reason. Even a cut-down rifle/shotgun is less concealable than an actual handgun, but... politicians generally know nothing about the things they're trying to ban.
Why putting a stock on a gun with a barrel that is under 16 or 18 inches requires registration hasn't made sense for decades, the Democrats are an anti-gun party and have been since their inception, but the Republican party hasn't had any interest on rolling back gun regulations since got the house back in 1994 even when there was a Republican president.
I would rather have the courts strike down the silly parts of the NFA act because if the GOP did de-regulate SBR/SBS/Suppressors the Donks would just outright ban them in a future law once they took power.