Also, do the suppressors have to be made in such a way that they can't be disassembled? I was thinking of threading the muzzle-end cap.
Federal law says that silencer parts are themselves silencers, but this does not mean they have to be permanently sealed. Most silencers for high powered rifles have welded end caps. This means they can be made with thinner tubing.
It is a good idea to have removable end caps and internals for anything using rim fire or cast ammo so that it does not fill up with gunk.
The Special occupational Tax (SOT) that FFL's pay is $1000 per year to make firearms other than destructive devices. It is $500 a year for those that only do a half million per year or less in business each year.