FailZero M16/M4 Bolt Carrier Group, No Hammer, EXO Nickel Boron Coated, Black Finish FZ-M16/4-01-NH-BLACK This Bolt Carrier Group (BCG) comes fully assembled and ready to drop into most mil spec M16" M4 or AR15 upper receivers. All critical parts are coated with EXO Nickel Boron technology that provides permanent dry lubricity" extreme durability and increases both wear and corrosion resistance.