The FailZero Lightweight AR-15 Bolt Carrier Group (BCG) comes fully assembled and ready to drop into most mil spec 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. Features21% Lighter than Mil-Spec M16Lower Mass Reduces Felt RecoilNickel Boron treatedExtreme durabilityDry lubricityCorrosion resistant