You can get approval to make some NFA weapons. A short barrel rifle, SBR, is a NFA weapon. You just go on the BATFE E Forms site, fill out a Form 1, upload your documents, and pay $200. It takes about thirty days to get approval. It comes in the form of a Tax Stamp, like the old Britishicon Crown Tax stamps back in the colonial days.

I just submitted one for a suppressor. They are also governed by the NFA.

Any Other Weapons, AOW’s also require a tax stamp. Things like a vertical forward grip on a pistol are AOW’s. The tax is only $5 for a vertical grip.