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by Bob, from KY, United States Asked on November 6, 2020
....from CMMG website,. "Barrel and BCG Kit, 5" 4140CM, SBN, 9mm
CMMG's 9mm 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, RDB uses a bolt that is forced to rotate and unlock, slowing the BCG. This results in a lighter BCG for a lighter rifle, and less felt recoil than a straight blowback system.
Our Barrel and Bolt Kits are designed to work exclusively with mil-spec AR-15 upper receivers with a modified ejection port, like our MkG/MkGs upper, and most mil-spec AR15 lowers when used with 9 AR Conversion mags as well as our MkGs Mk17 and Mk9* lowers. Other manufacturers’ receivers may work, but we cannot guarantee function or reliability.
*Older Mk9 lowers will require one of our retrofit kits to convert it to a Radial Delayed Blowback compatible lower, instead of Traditional Blowback.". (Older Mk9 lowers are Colt SMG, so, a conditional Yes )