The Chrome Moly AR-15 HBAR contour barrels feature a gas port drilled for a .750 gas block. 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 prior to use. Include 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 A(R) Steel; HPT (High Pressure Test/Proof); MPI Bolt (Magnetic Particle Inspected); Shot Peened Bolt; Tool Steel Extractor; BCM A(R) Extractor Spring; Black Extractor Insert; Mil-Spec O-Ring.