One of the more under-appreciated aspects of the U.S. Constitution is it's forbidding of ex post facto laws where laws are passed making previously legal behavior illegal.
Given this, I can see where it might be argued that NFA 1934 was permitted due to there largely being no such gun laws beforehand, but how would follow-on efforts in '68, '86, and '92 not run afoul of ex post facto? My grandfather, for example, had a fully-automatic Thompson that he was forced to get rid of prior to the '68 legislation because he didn't want to pay the tax to register it. Given that he bought it when it was fully legal, how was the registration requirement not contrary to that aspect of the Constitution?
Given this, I can see where it might be argued that NFA 1934 was permitted due to there largely being no such gun laws beforehand, but how would follow-on efforts in '68, '86, and '92 not run afoul of ex post facto? My grandfather, for example, had a fully-automatic Thompson that he was forced to get rid of prior to the '68 legislation because he didn't want to pay the tax to register it. Given that he bought it when it was fully legal, how was the registration requirement not contrary to that aspect of the Constitution?