The WMD Bolt Carrier Group is precisely machined from high-grade steel to semi-auto specs before receiving WMD's proprietary NiB-X nickel boron coating. This coating results in super-smooth action extreme corrosion resistance and a harder-than-chrome surface that prevents carbon fouling from adhering making this an ideal choice for any one who does high-volume shooting. This set features a polished finish and includes a matching NiB-X coated hammer for the smoothest cycling and trigger action. This set features a Polished Finish and includes the Hammer. - Caliber: 5.56 NATO - Bolt Finish: NiB-X Polished - Bolt Material: Carpenter No. 158 - Carrier Finish: NiB-X Polished - Carrier Material: 8620 Steel - Extractor: O-Ring - Gas Key: Staked