The AR-STONER AR-15 Side Charging Pistol Upper Receiver Assembly Gen 2 features a 7.5? barrel with 7? Ultralight M-Lok handguard. The 7.5? barrel is a M4 contour, made of 4150 Chrome Vanadium with a matte finish. It also features a 1:7 twist rate, and a pistol length gas system. The upper includes a 7? M-LOK rail, a bolt side charging upper carrier