Perfect for building your custom AR15 upper group. Comes with properly installed M4 barrel extension. All barrel sub-assemblies are properly headspaced. - 5.56x45 NATO Chamber - 12.5 - Multi Role Profile - 4140 Chrome Moly - Salt Bath Nitride Finish inside and out - 1:7 Twist - Threaded 1/2-28 - Carbine length gas port - .750 Gas Journal