The whole "it's a firearm....., wait, no it is just an accessory--wait, no really it's a firearm! Just kidding, no it's not..." thing is so mired up in the courts and regulations that no one really knows the best way to proceed on that point. We just saw a 5th circuit opinion reversal on that very thing.
The original question here was now what that the tax is zero and thus the registration part is now conspicuously hanging in the breeze. It's a problem, and a big one. Originally, the NFA could only be passed as a tax, kind of a loophole in the system that then required registration as proof of that tax paid and without it, you were in trouble with the IRS. Any other way and the NFA would have been a direct 2A violation and congress knew that very well at the time. Now that the tax is zero, well, all we have is the unconstitutional registration requirements which are blatantly illegal.
We'll just have to keep chipping away at the problem through our juggernaut of a system little by slowly from all angles until this shakes out in a more constitutional manner. I think this can still be fixed to restore constitutional freedom, but it is going to take a marathon effort to elect and appoint people who have the fortitude to stick their necks out and actually follow through on their sworn oath to "support and defend the Constitution against all enemies, foreign and domestic, to bear true faith and allegiance to it, and to faithfully discharge the duties of their office."