RCA 5.56 Black Nitride Bolt Carrier Group M16 - Noveske marked with a black nitride finish. Carrier features 8620 steel with 158 Carpenter Steel bolt and 9310 tool steel extractor and cam pin. - Much more than typical Parkerizing phosphating or black oxide Blacknitriding is a thermal chemical diffusion of nitrogen and carbon into the surface of the steel. This process increases wear-resistance fatigue strength corrosion resistance lubricity and cosmetic appeal.