Sounds like you want a 16" or 18" barrel with a mid or lightweight profile. If possible go with a rifle-length gas system (16" w/ rifle gas works just fine on .308s). You may have to have a barrel re-profiled to get what you want (I did with my MWS' barrel). Since you're running suppressed, an adjustable gas block might be good instead of the heaviest buffer you can find, and it'll be lighter weight. Definitely go with an adjustable stock system and use AR-10 pattern parts (longer receiver extension, AR-10 spring, and AR-15 adjustable stock sized buffers). Both of my .308 ARs have VLTOR A5 7-position receiver extensions (same size as AR-10 pattern RE), H3 buffers, and Magpul ACS-L stocks. You're also going to want to reduce the weight in front of the magwell as much as possible for better handling. When I configure my PWS MK216 and LMT MWS to weigh the same, the PWS still noticeably handles better due to less weight out front (the MWS has a thick barrel). The lightest rails for an LR308 I know of are the Midwest Indsutries SS, Samson Evolution, and Fortis Switch. Neither of my .308 ARs needed break in for things to run, but the MWS' BCG is hard chromed and the MK216's is polished from the factory (and glides SUPER smoothly through the upper). A coated BCG probably won't encounter any noticeable break-in effects, nor would a quality one I suspect.