The Troy Industries Di-Optic Aperture (DOA) is the future of flip-up iron sights; with an advanced design, superior sighting speed, and battle-ready toughness. These sights provide lightning fast target acquisition and are manufactured to exceed military specs. The optically engineered apertures center your eyes instantly and effortlessly providing a clearer target picture than with round apertures.The Troy DOA sights feature two settings, one for up to 300 meters and a second for up to 500 meters. This versatility makes the DOA ideal for military, law enforcement and home defense applications. Notes: The measurement to the center of the standard aperture is 1.36"The standard sights are meant for Mil-spec top railsSight is designed to work on same-plane rail systems only. Will not work with a railed gas block that is higher or lower than the receiver. Front sight must be mounted at the same height plane as the rear sight. Railed gas blocks that are lower than receiver height w...