CMMG MKGS Barrel, 9MM, 5 Barrel, Black Finish, Includes BCG, Fits AR Rifles 99D17C3 The CMMG 5" 9MM MkGs Barrel and BCG Kit allow you to build a Pistol Caliber Carbine using their patent-pending Radial Delayed Blowback System. Unlike typical direct blowback systems, their RDB uses a bolt that rotates and unlocks, slowing the BCG. This results in a lighter BCG for a lighter pistol or rifle, and reduced felt recoil compared to a straight blowback system.