What does the Firearms Act actually tax?

All gun control laws are derived from The National Firearms Act of 1934, as amended. The National Firearms Act of 1934 is a tax law. But a Constitutionally guaranteed right cannot be taxed — so what does the Firearms Act actually tax? I don’t know the answer. Please help me understand this. I suspect that it is the right to use firearms in regulated Commerce (see the Supreme Court’s Lopez case) but I cannot confirm this. 

“All laws which are repugnant to the Constitution are null and void.” — Marbury v. Madison 


“… the performance of duty by constituted officers must not be thwarted. But these agents, servants of a Government and a society whose existence and strength comes from these constitutional safeguards, are serving law when they respect, not override, these guarantees. The claim and exercise of a constitutional right cannot thus be converted into a crime.”
— US Supreme Court in Miller v. US, 230 F2d 486, 489 (1956).

“… reflect the obvious concern that there be no sanction or penalty imposed upon one because of his exercise of constitutional rights”
— Sherer v. Cullen , 481 F2d 945.

Murdock v. Pennsylvania, 319 U.S. 105 (1943), The Supreme Court ruled that

“The state cannot and does not have the power to license, nor tax, a Right guaranteed to the people,”
and
“a license tax—a flat tax imposed on the exercise of a privilege granted by the Bill of Rights. A state may not impose a charge for the enjoyment of a right granted by the federal constitution. Thus, it may not exact a license tax for the privilege of carrying on interstate commerce (McGoldrick v. Berwind-White Co., 309 U.S. 33, 56—58, 60 S.Ct. 388, 397, 398, 84 L.Ed. 565, 128 A.L.R. 876), although it may tax the property used in, or the income derived from, that commerce, so long as those taxes are not discriminatory. Id., 309 U.S. at page 47, 60 S.Ct. at page 392, 84 L.Ed. 565, 128 A.L.R. 876 and cases cited. A license tax applied to activities guaranteed by the First Amendment would have the same destructive effect. It is true that the First Amendment, like the commerce clause, draws no distinction between license taxes, fixed sum taxes, and other kinds of taxes. But that is no reason why we should shut our eyes to the nature of the tax and its destructive influence. The power to impose a license tax on the exercise of these freedoms is indeed as potent as the power of censorship which this Court has repeatedly struck down. ”

In another 1st Amendment case, Shuttlesworth v. Birmingham 394 U.S. 147 (1969), the Supreme Court ruled similarly, that

“And our decisions have made clear that a person faced with such an unconstitutional licensing law may ignore it and engage with impunity in the exercise of the right of free expression for which the law purports to require a license. ”

So what does the Firearms Act actually tax?

 

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