CMMG MK57 NO FURTHER A MYSTERY

With a number of the distinctive projectiles I'd useful — most notably the large, 55 grain subsonic rounds from Detroit Ammo Co (not pictured) — the bullet-formed grooves inside the entrance from the FN journal didn’t play nicely.The projectiles would adhere or drag in there and also the follower would bind. Since the ProMags have no inward-p