The Armaspec .223/5.56 AR-15 bolt carrier group features a durable black Nitride finish which improves hardness and reduces friction making for an extremely smooth running BCG that is easy to clean and maintain. With Mil-Spec dimensions, this AR-15 bolt carrier group is compatible with any AR-15 upper receiver and can handle .223 Remington, 5.56 NATO, and even .300 blackout ammunition. If you are looking for a high quality Mil-Spec BCG with an improved coating, you can't go wrong with Armaspec.