Cryptic Bolt Carrier Group BCG - Complete 6.5mm Grendel II 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. HPT (High Pressure Batch Tested) and MPI (Magnetic Particle Inspected). Specifications for Cryptic 6.5mm Grendel II 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: 6.5mm Grendel II Gun Make: AR Platform Gun Model: M16 M4 AR-15 Hand: Ambidextrous Weight: 11.55 oz Condition: New Additional Features: Thickness - 2-3 microns Included Accessories: Standard Features of Cryptic 6.5mm Grendel II Bolt Carrier Group BCG - Complete Reduced Carbon Fouling; Easier Cleaning; Full Auto Carrier (Backward compatible with all semi-auto rifles) Package Contents: Cryptic 6.5mm Grendel II Bolt Carrier Group - Complete Gas Key Key Staked with Grade 8 Screws Bolt w/ Spring and Ejector Extractor w/ Spring and viton O-ring Firing Pin Cam Pin What firearm models does the manufacturer say Cryptic 6.5mm Grendel II Bolt Carrier Group (BCG) - Complete is compatible with? The manufacturer suggests that Cryptic 6.5mm Grendel II Bolt Carrier Group (BCG) - Complete is compatible with AR-15 M4 M16Cryptic CC-1200: Bolt Carrier Group - Complete 6.5mm Grendel II Mystic Black