Taxing a right is unconstitutional. Whether the courts will rule that way is a different story.
There are two types of taxes: direct and indirect. Direct taxes apply only to slaves and land. Indirect taxes are privilege taxes and the tax can be avoided by not engaging in the activity. Such taxes cannot be applied to rights, though there seems to be a push to apply such taxes via abuses of the Commerce Clause.
The basis for firearms regulation comes from corruption of the Commerce Clause that started in the 1930's. The Federal government has decided that the Commerce Clause can be used to nullify and/or control rights and expand Congressional power beyond the enumerated powers listed in the Constitution. The first line of the Gun Control Act of 1968 reads:
An Act to amend title 18, United States Code, to provide for better control of the interstate traffic in firearms.*
Here is what cracks me up:
TITLE I - STATE FIREARMS CONTROL ASSISTANCE
Sec. 101. The Congress hereby declares that the purpose of this title is to provide support to Federal, State, and local law enforcement officials in their fight against crime and violence, and it is not the purpose of this title to place any undue or unnecessary Federal restrictions or burdens on law-abiding citizens with respect to the acquisition, possession, or use of firearms appropriate to the purpose of hunting, trapshooting, target shooting, personal protection, or any other lawful activity, and that this title is not intended to discourage or eliminate the private ownership or use of firearms by law-abiding citizens for lawful purposes
, or provide for the imposition by Federal regulations of any procedures or requirements other than those reasonably necessary to implement and effectuate the provisions of this title.
And, yet, it does place restrictions upon law abiding citizens. Citizens in various Federal territories, including the District of Columbia, find it nearly impossible to acquire firearms because of the Gun Control Act of 1968.
Back to taxation: if it's not a right, then it is a privilege. If it is a privilege, then it can be taxed as an indirect tax. People can avoid the tax by not engaging in the activity. Therefore, one cannot be taxed for arms and ammunition if one does not have any arms or ammunition.
I do not doubt this is their goal. The state of Illinois just floated this idea in its legislature. The tax can start out at 0.000000000001%, but the very next day the legislature can raise the tax to 100%, 1000% or any level they so desire. This was the intent of the $200 NFA tax because that amount of money at the time the law was passed was obscene. It effectively destroyed private ownership of fully automatic firearms.
So, yes, taxation of firearms and ammunition is to be avoided at all costs.
* Citation: Gun Control Act of 1968