This 6mm ARC Grendel Bolt Carrier Group (BCG) comes fully assembled and ready to drop into most Mil-Spec upper receivers chambered in 6mm ARC. All critical parts are coated with EXO Nickel Boron technology that provides permanent dry lubricity extreme durability and increases both wear and corrosion resistance. EXO Nickel Boron makes for easy cleanup and will improve the performance of your rifle.Nickel Boron treated parts include: 6mm ARC Bolt & Extractor M16 Cut Bolt Carrier Cam Pin Carrier Key Firing Pin