Re: 308 BCG
Well, the D is the DPMS platform, so you have:
1) DPMS (factory original, well-made, cheaper than other alternatives, available in chrome or phosphated finish for $290-310 complete);
2) Fulton Armory (nice units, but still pricier than DPMS at $350, chromed only, good quality froma reputable mfg)
3) JP Enterprises (low mass carrier w/ enhanced bolt, expensive as hell at roughly $450, but functions 100%).
FWIW, I believe if you contact IRA they'll tell you they are using standard factory DPMS offerings with good success. While any of the above will probably work, I would spend the money on the DPMS and use the savings for the remainder of your build.