With the various rail systems currently being offered for the AR15/M16/M4 weapon system, the Vltor Low-Profile Gas Block is small enough to fit inside most rail systems.The Vltor Low-Profile Gas Block covers one-inch of the barrel's surface and is made to the tightest tolerance. These gas blocks are offered in 17-4PH stainless steel, in matte Black or matte Stainless finish. Uses two socket head (allen) cap screws clamp to barrel. Fits barrels with a diameter of 0.625 inches. Gas tube roll pin included.