AR-STONER AR-15 Barrel chambered in 450 Bushmaster. Constructed from Chrome Moly Steel, this button-rifled barrel features a heavy contour and a 1:24 twist. Includes M4 feed ramps and a 5/8-32 muzzle thread. Barrel extension and locator pin are included and installed at the factory. 18 inch length for rifles designed for 450 Bushmaster.