The LMT e bcg.
In the simplest terms, the third hole letting out more gas. It's not what it was originally intended for (overgassed), but it works well in most suppressed SBRs. Some people will also change out the buffer, but the standard has worked well for me in my 10.5" LMT.
http://www.lmtstore.com/parts/bolts-carriers-and-groups-complete/556-enhanced-fa-bolt-carrier.html
