AR-15 5.56/.223 BA Bolt Carrier Group (BCG) Complete Assembly Nickel Boron. This BCG is the real deal. Serrated. This assembly is made to last and to perform in the worst conditions. Staked screws, nickel boron plated inside and out. High lubricity. Highly wear resistant. Standard gas key is designed to fit gas tubes with an outer diameter of .172" to .179". This BCG has been tested by the US Army and fits standard M4s.