Nickel Boron AR15 Bolt Carrier Group Precision machined complete BCG (Bolt Carrier Group) combines shot peened surfaces with Nickel Boron finish for the best lubricity, function, and endurance. Fits guns chambered for .223, 5.56, .204, 300 Blackout, etc. Made to exact compliance with military drawings, then taken to the next level with advanced machining and finishing processes: The bolt is 9310 formula steel, heat treated, magnetic particle inspected, with gas rings fitted to Milspec. The carrier is 8620 formula steel, heat treated, and held to the closest internal tolerances for accuracy and reliable function... Including full auto applications. The cam pin and firing pin are Swiss Screw turned and precision ground to achieve the closest dimensional tolerances and straightness. CNC machining and exact heat treating makes the extractor reliable and extremely wear resistant. And finally the Gas Key... CNC machined to comply exactly with military drawings, it gets heat treated, chrome lined, and is then torqued and staked onto the carrier in compliance with full auto military specifications. If you are shopping for an accuracy enhancing AR15/M16/M4 Bolt Carrier Group, you can't buy better than this one... At ANY Price!