Nordic Components heavy barrel 11.5". Made from 4150 (machine gun steel) with a QPQ finish and a 1/7 twist. A carbine gas system for reliability. The nickel boron barrel extension has M-4 feed ramps and chambered in 5.56. Includes installed FSB, Barrel Nut, And Endcap. Select F-Marked for a flat top upper, A2 if you have a fixed carry handle.