The Aero Precision 5.56 Bolt Carrier Group for AR-15 rifles is made from high quality materials with quality assurance measures like high pressure testing and magnetic particle inspecting. The bolt assembly is machined from 9310 steel and tested to ensure it can withstand thousands of high-pressure rounds. The Carrier assembly is machined from 8620 steel and the entire BCG is magnetic particle inspected to ensure every dimension is as precise at can be. The black Nitride finish increases hardness and creates an extremely smooth surface for ease of cleaning and improved reliability.