The NFA tax stamp is at its lowest relative cost ever. It began at $200 and never was indexed for inflation; it would be considerably more today if it had been. IMO, the biggest hurdles to NFA ownership have to do with the extensive wait time (5-6 months for approval) and for full autos, the 922o registry closure.
Then, you have various states with a patchwork of laws; what's legal in one state may not be one state over, federal NFA paperwork or not.
IMO, if they just repealed 922o and sped up the approval process to one month, the $200 taxes would pile up rather quickly, and our government would have some nice revenue.