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).