Centurion
Super Freak
+1 for the FNH forums.For any NFA / Suppressor related information I recommend checking out the FNH forums. I've been spending more time there than other forums as I've found it's a very friendly and mature population of users that are open to providing information. Many of the vendors are active on that forum, SilencerShop employees in particular, and are quick to answer a lot of questions without attacking people.
SBR, Suppressors and NFA Items
(None of them claim to be attorneys or ATF employees, but they just share what they did and worked for them when they embarked on this process)
I use to be an active participant in their SCAR threads.