FailZero Bolt Carrier Group With Hammer, Completely Assembled, EXO Coated, Fits M16/4, Nickel Finish FZ-M164-01-SAH 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.