FN AR15 barrels are hammer forged using the same proprietary high-grade Chrome Moly Vanadium steel used in FN's venerable machinegun barrels produced to handle the punishment of sustained fully automatic fire. After forging, each barrel is Magnetic Particle tested 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 5.56x45mm in 14.5, Carbine, 16, Carbine, 16, Mid-Length, and 18, and 20, Rifle Length gas systems.