[OneWingedShark]

Another point to consider is the case of US v. Rock Ridge Armory wherein the judge ruled against the ATF precisely because their "regulatory powers" stemmed from the ability to tax, and that by withholding the tax-stamp for new machine-guns they could not legitimately regulate machine-guns. -- This case's reasoning was so strong the ATF refused to appeal it higher because if they did the ruling [if upheld] would have stripped them of power in multiple states.

In addition to this, the 2nd amendment applies to the original Constitution "in order to prevent misconstruction or abuse of its powers, that further declaratory and restrictive clauses should be added" and therefore applies restriction to taxes 'infringing' the rights of keeping & bearing arms... AND to the commerce clause.
There might be argument that the 16th Amendment could be used to apply against weapons at the point-of-sale, but that has not been argued as the power/authority by which the right is regulated.

[BCR #226]

Another case to look at is US v. Leary. When the Government pulled the same stunt on marijuana and Dennis Leary handed their butts to them in the late 60’s. The MG ban is very similar in wording as the marijuana ban then.

Those tax laws are a pain sometimes.