There is, in fact, a good question wrt how much of NFA is actually constitutional. A funny thing about this is that ATF has dropped cases on several occasions where the defendant tried this angle, presumably because they didn't want something contrary to their current regulations as written to be overturned in court, and because they had plenty of other charges to throw at those guys anyway.

Given that it was modeled on gun control legislation from pre-WWII Germany, it's _not_ something which they want looked at too closely.