Match-quality Nitromet™ 5.56 Barrel 16″ GP barrel, has been treated with Nitromet™ (salt bath nitrocarburized) inside and out with a hardened finish that results in a bore life 40% greater than with traditional chrome-lining without affecting accuracy. Barrels are made of 4150 Chrome Moly Vanadium steel, chambered for 5.56mm NATO, button rifled with 1-8″ twist rifling, and machined to a standard M4 profile. Gas port drilled for a carbine length gas system, .750″ diameter at the gas block. . Comes with a Nitromet™ processed barrel extension installed.