I've seen similar things for sale at local gunshows, only the part that need be "stamped" is the adapter according to the seller ... I don't know how true that is as I have built a couple form1 suppressors and I haven't read it anywhere, but have been told that once it's made only a licensed smith can open it and "alter or repair" it ... That doesn't make sense to me, but it doesn't suprise me either.
It's true. A form 1 builder cannot legally replace components, and certainly no spare parts. It's stupid, but that's the rules. Of course plenty of people do, and it's not a problem for them as long as they don't go posting about it on public boards or social media, but technically they are breaking the law.
because, if that's the case, why couldn't I put different end caps with different threads on my suppressors?
You can, sort of. You just need to be sure you call it a mount, not an end cap. Prevailing wisdom is that if the rear mount does not serve to retain the baffle stack, it's just a mount, not a suppressor part. That's why you can buy direct thread & 3 lug mounts for pistol cans that normally have a booster piston in them, and all manner of 1-3/8 24 TPI mounts to fit a number of different rifle cans from various manufacturers. I routinely modify people's suppressors to accept this new de facto standard, hacking off proprietary or direct thread mounts and threading them 1-3/8 24 for Q plan B, SiCo ASR, YHM #3060, etc.
On front caps, it's much more grey. On the one hand, a front cap is technically a suppressor part. On the other, there are a number of commercial suppressors out there you can buy different caliber front caps for directly from the manufacturer, including designs where the cap retains the stack like the Rugged Obsidian 45.
How does that translate to these oil filter adapters and the like? Technically, you would register the serialized mount and assemble it yourself on a F1 the first time, then have to have one of us SOTs remove the old filter or bottle and screw a new one on each time it needed replaced. Practically, I don't think one would need to worry about JBTs kicking in their door over putting that new small block oil filter on the mount they went through the hassle and expense of registering in the first place. That said, oil filters and soda bottles are not good suppressors. You'll get a little SPL reduction, but it'll be very disappointing, and anything more than 9mm is gonna blow them apart.
