Obsidian Arms M16 Bolt Carrier Group BCG 5.56 Phosphate Black. 56 bolt. Specifications for Obsidian Arms M16 Bolt Carrier Group (BCG): Manufacturer: Obsidian Arms Color: Black Finish: Phosphate Fabric/Material: 8620 Steel Caliber: 5.56x45mm NATO Gun Make: AR Platform Gun Model: M16 AR-15 Gun Type: Rifle Hand: Right Country of Origin: USA Features of Obsidian Arms M16 Bolt Carrier Group BCG M16 Profile Machined from 8620 steel with forward assist serrations Mil-Spec phosphate coating with chrome lined bolt channel Assembled with: Properly staked Nitride gas key 5.56 Bolt is machined from 9310 steel Shot peened and Phosphate finished Black o-ring insert on extractor Enhanced AR-15 Firing Pin and Cam Pin Package Contents: Obsidian Arms M16 Bolt Carrier Group What firearm models does the manufacturer say Obsidian Arms M16 Bolt Carrier Group (BCG) is compatible with? The manufacturer says that Obsidian Arms M16 Bolt Carrier Group (BCG) is compatible with AR-15 M16Obsidian Arms OA-M16-BCG-P-556-N: M16 Bolt Carrier Group 5.56 Phosphate Black