If you want to make your own, you can. First you get permission, and the tax stamp. Then you make A silencer. It turns out to be a crappy one, that is not very quiet? Too bad, so sad. You want to try again? That requires another 200 dollar tax.
Your question mentions two entirely different things.
>if you've paid the tax and have the license,< The tax is for individuals on each NFA item. It is NOT a license.
The license is for a company that manufactures.
If you want to make them for sale, you need to get a license. If you want to own one (or dozens) for your own use, you need to pay a tax on each and every one.
Two different things.
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.