The prohibition is legally invalid: United States v. Rock Island Armory
Unfortunately this legal precedent is ignored in the legal-world, because to acknowledge it would be to cut off a huge chunk of the power they have over firearms... that, in turn, could also lead to some serious paring down of powers in other arenas the Government has over-reached in.
Alas, the ruling’s nuance was that they can’t prosecute you for not paying a tax if, by law, they can’t accept it. Portions of NFA ‘34 are rendered impotent by the existence of 922(o). The case did not invalidate 922(o), and if 922(o) falls then relevant parts of NFA resume effect.