Perfect for building your custom AR15 upper group. Comes with properly installed M4 barrel extension. All barrel sub-assemblies are properly headspaced. – 5.56×45 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