IF Congress and the Supreme Court actually cared about the Constitutional limits on the Federal Government, you could own any cannon, MG, RPG, etc you wanted without the $200 tax.
Strictly speaking, the 2nd Amendment does not forbid the Feds from prohibiting non-military weapons! US v Miller specifically addressed that question and the result was a determination that only militarily-useful weapons were protected.
In DC v Heller, the Justices absolutely refused to address the issue of machine guns and other military weapons. Instead, they used convoluted language to say that the 2nd amendment means an individual right, but that existing laws against MGs, etc are OK.
The Supreme Court ruled many years ago that a sales tax on newspapers was an infringement on the 1st amendment. But, they somehow feel that a $200 tax on old guns, and outright prohibition on new military-grade firearms is not an infringement. Explain the logic in that?