The new PRi forearm rail mounted front sight is designed just like the original barrel mounted flip up front sight. The rail mounted sight works on any rail that is mounted at the same levels as the receivers rail. The sight then can be raised and locked in the up position for iron sight use. USES STANDARD M-16 FRONT SITE POST. This sight will NOT work on gas blocks unless your gas block is built to the same height over bore as the flat top receiver.