CMMG Barrel and BCG Kit, 5.7X28MM, 8 Barrel, Fits AR Rifles, Black 57D88C3 The 8" 5.7x28mm Barrel and BCG Kit is perfect for building an AR pistol or SBR in 5.7x28mm using CMMG's patented Radial Delayed Blowback operating system. Unlike typical direct blowback systems, Radial Delayed Blowback not only reduces the weight in your BCG but it results in less felt recoil than a straight blowback system and still provides essential features like last round bolt hold open. CMMG's 5.7x28mm Barrel and Bolt Kits are designed to be assembled into a Mk4/mil-spec AR15 upper and work with either a CMMG Mk57 lower receiver or Mk4/AR15 mil-spec lower receiver (when using CMMG 5.7 AR Conversion Magazines).