FN AR-15 Hammer-Forged Barrels have a chrome-lined chamber and bore to help resist corrosion and provide heat erosion resistance. Their barrel is forged from Chrome Moly Vanadium steel and built for the .750" diameter gas block with 1/2x28" threaded muzzle. It has an M4-style feed ramp and barrel extension for consistent and reliable feeding. This barrel is chambered in 5.56 NATO with a 1:7" twist with a manganese phosphate finish.