CMMG .40 S&W Barrel and BCG Kit 5" 4140 Chrome Moly, Nitride Finish MkGs Barrel and BCG Kit allow you to build a Pistol Caliber Carbine using this Radial Delayed Blowback System which 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. Designed to work exclusively with CMMG MkGs Lowers. Other manufacturers lowers may work, but are not guaranteed for function or reliability. Specifications: 5? Medium Taper 1:16 Twist 4140 Chrome Moly Salt Bath Nitride Finish inside and out 40 S&W Chamber Threaded .578-28 Radial Delayed Blowback Bolt Requires enlarged ejection port for reliable operation. Weight: 1.7lbs Installing this product will subject it to NFA laws and doing so without a license is a felony possession of a Class III firearm. Check your local laws.