Cryptic Bolt Carrier Group BCG - Complete .223/5.56/.300 AAC Blackout Left Hand Mystic Black. BCG includes the proper chrome lined and heat treated gas key. Gas key is secured to carrier with Grade 8 fasteners and properly staked per Mil-Specs. This ensures a strong staking and proper gas system functioning. The bolt assembly is CNC machined from Mil-Spec Carpenter No. 158 steel and shot peened for increased strength per Mil-Spec; includes tool steel extractor with spring and viton O-ring; firing pin chromed finished with firing pin retaining pin and cam pin. HPT (High Pressure Batch Tested) and MPI (Magnetic Particle Inspected). Specifications for Cryptic .223/5.56/.300 AAC Blackout Left Hand Bolt Carrier Group (BCG) - Complete: Manufacturer: Cryptic Finish: Chrome Plated Fabric/Material: 8620 Aircraft Quality Steel Carpenter 158 Steel Carrier Material: 8620 Steel Bolt Material: Steel Caliber: 5.56x45mm NATO .300 AAC Blackout .223 Remington Gun Make: AR Platform Gun Model: M16 M4 AR-15 Hand: Ambidextrous Weight: 11.55 oz Condition: New Included Accessories: Standard Features of Cryptic .223/5.56/.300 AAC Blackout Left Hand Bolt Carrier Group BCG - Complete Reduced Carbon Fouling; Easier Cleaning Package Contents: Cryptic .223/5.56/.300 AAC Blackout Left Hand Bolt Carrier Group - Complete Chrome Lined Gas Key Key Staked with Grade 8 Screws Bolt w/ Spring and Ejector Tool Steel Extractor w/ Spring and viton O-ring Firing Pin w/ Chrome Finish Cam Pin What firearm models does the manufacturer say Cryptic .223/5.56/.300 AAC Blackout Left Hand Bolt Carrier Group (BCG) - Complete is compatible with? The manufacturer says that Cryptic .223/5.56/.300 AAC Blackout Left Hand Bolt Carrier Group (BCG) - Complete is compatible with AR-15 M4 M16Cryptic CC-0100-LH: Bolt Carrier Group - Complete .223/5.56/.300 AAC Blackout Left Hand Mystic Black