Replacement Low Profile Gas Block for the SR-15/SR-16 series. This block has a 0.750" inside diameter and is secured by a pair of set screws, making it a good choice for use with the URX 4 rail. Also compatible with other KAC URX rail systems. This gas block is used on the Mk12 Mod 1 rifles used by the US military. Mfg: Knights Armament SPECS: Fits - AR15/M16 barrels with a .750 diameter at the gas shoulder.