CMMG Barrel and BCG Kit 16in 4140CM SBN 40 S&W. 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. Specifications for CMMG .40 S&W Barrel And Bcg Kit: Manufacturer: CMMG Finish: No Fabric/Material: 4140 Steel Caliber: .40 S&W Barrel Profile: Medium Tapered Threads per Inch: .578x28 Barrel Twist Rate: 1-16 Flutes: No Gun Make: CMMG Weight: 1.4 lb Package Contents: CMMG .40 S&W Barrel And Bcg Kit CMMG 40D98DE: Barrel and BCG Kit 16in 4140CM SBN 40 S&W