FN America Barrel, 18, 556NATO, 1:7 Twist, Button Broached, Rifle Length Gas System, M16 Contour, Fits AR-15, Matte Finish, Black 20-100042 FN AR15 premium button rifled barrels are are manufactured using U.S. sourced Chrome Moly Venadium material per Mil-B-11595E. Each barrel is High Pressure Tested (HPT) and Magnetic Particle Inspected (MPI) to ensure that it is free of micro-stresses or flaws, resulting in consistency and quality in every barrel shipped from the factory. The bore and chamber are chrome lined to help resist corrosion and provide resistance to heat erosion which greatly improves barrel life and reliable chambering / extraction. The M4 style feed ramps and barrel extension provide consistent feeding and the matte black Manganese Phosphate finish resists external corrosion and oxidation. Currently available in a chambering of 5.56 x 45mm in the following profiles and gas system lengths 10.5" Government Carbine, 14.7" Government Midlength, 14.7" M4 Carbine, 16" M4 Carbine, 16" Government Carbine, 16" Government Midlength, 18" Government Rifle, 20" Government Rifle.