The CMMG40 S&W MkGs Barrel and BCG Kit allow you to build a Pistol Caliber Carbine using our Patent Pending Radial Delayed Blowback System. Unlike typical direct blowback systems, our RDB uses a bolt that is forced to rotate and unlock, slowing the BCG. This results in a lighter BCG for a lighter pistol or rifle, and less felt recoil than a straight blowback system.Our MkGs Barrel and Bolt Kits are designed to work exclusively with our MkGs Lowers. Other manufacturers' receivers may work, but we cannot guarantee function or reliability.**All NFA rules apply** See linked CMMG Resource article for more information.All CMMG products come with our Lifetime Quality Guarantee.