300BLK Build Questions

I'm currently in the process sourcing parts to build a 300 BLK AR pistol. Right now I have a Griffin Armament MK2 AMBI AR15 Lower Receiver & a SB Tactical HBPDW 556/300BLK. I'm planning on running a Ballistic Advantage Rigid 6in 300 Blackout AR 15 Barrel, undecided on an upper receiver. Gun will be shot suppressed 100% of the time. My question is what BCG should I be looking at? Standard Mil Spec or something else and should I get phosphate coated? Any input or advice would be greatly appreciated!