Our Low profile Quad rail gas block is constructed of Anodized Aircraft grade aluminum with a Matte Finish. The Low profile gas block replaces the existing A2 front sight. It comes equipped with 4 picatinny rails allowing you to mount accessories such as a front sight or a laser underneath. The top rail is lower than the flat top by 0.28". It is compatible with .308 with a .936 diameter barrel. Roll pin & set screws are included. No modification to your barrel is required.