Rosco Manufacturing Bolt Carrier Group, 556NATO/300 Blackout, Fits AR-15, Phosphate and Chrome lined Finish, Black ROS-BCG-002 These 5.56 / 300 BLK Bolt Carrier Groups are Phosphate Coated with Chrome Lined Gas Key & Bolt Channel" Full-Auto rated" properly staked" and the bolts are Matched to the barrel. Their Rosco Phosphate Bolt Carrier Groups are designed to meet the modern-day carbine shooters requirements. For their Phosphate Bolt Carrier Groups - they choose to team up with the best - Microbest" so the end user has dependability time after time" every time. They have also balanced input from their consumers and military end users in order to bring you the product you need" not just the one they want to sell you. These BCG's are to MIL Specification and have a properly staked gas key along with a chrome lined coating for extended performance. They also feature a Shot Peened/ MP tested Matched bolt (with upgraded extractor) which is designed to lock up perfectly with their extensions for extreme accuracy. These BCG's are made to withstand Full-Auto rated fire and unforgiving use from the end user.