The Hybrid contour AR-15 barrels offer an ideal configuration for those seeking extreme accuracy without the added weight of an HBAR contour. An indent is provided opposite the gas port to enable easy installation. Also included is an M4 barrel extension. These barrels accept muzzle devices with a 1/2x28 thread. Chambers are finished, but barrels should be headspaced and installed by a qualified gunsmith. Included are a headspaced BCM Bolt Assembly with your Criterion chrome-lined barrel. The BCM Bolt is MPI tested, includes tool steel machined extractor, BCM Extractor Spring and is shot peened for increased strength. Mfg: Criterion Barrels Inc SPECS: Milspec Carpenter No. 158 Steel; HPT (High Pressure Test/Proof); MPI Bolt (Magnetic Particle Inspected); Shot Peened Bolt; Tool Steel Extractor; BCM Extractor Spring; Black Extractor Insert; Mil-Spec O-Ring, .750 ID".