Check out www.major-malfunction.com (may not be able to link from this site, just type the address in) he is a FFL and Cls III dealer that specializes in that kind of thing. If you have questions, call him, he is a good dude. Just tell him his army brother told you to call him. If he doesn't hang up, he will help you out. Don't feel like you have to buy from him, he will be glad to talk to you if you just want info. And he does have great prices if you want to buy (all NFA laws apply).
I've got an AAC M4-2000 Mod 7 .223 suppressor. It uses a Quick Attach ratchet type system that insures your can won't unscrew under use. I use it on my RRA 16" barrel, threaded 1/2-28 with a Phantom flash hider. The flash hider stays on the barrel and the can screws on over it. This can is FA rated. It makes it sound like a .22 rifle. I've gotten 4 cans and a SMG thru these guys. http://www.politesocietyinc.com/