Di-Optic Aperture (DOA) Fixed Rear Sight Advanced design, superior sighting and battle-ready toughness define Troy Industriesí new Di-Optic Aperture (DOA) fixed sights. Looking for rock solid stability and dead-on accuracy in a fixed sight? Troy Industries Di-Optic Aperture (DOA) fixed sights get the job done with an advanced design, superior sighting ability and battle-ready toughness.. This permanent, rail-mounted alternative to a standard A2 sight allows limitless mounting options on all-length rails with a serrated ramp to eliminate glare. Provides a continuous zero, unflinching durability and peace of mind. These sights provide lightning fast target acquisition and are manufactured to exceed military specs. The optically engineered Di-optic apertures center your eyes instantly and effortlessly providing a clearer target picture than with round apertures. The sights feature two settings, one for up to 300 meters and a second for up to 550 meters. This versatility makes the DOA ideal for military, law enforcement and home defense applications. These advanced sights have been extensively field-tested and, like all Troy Industries products, are Battle Ready. Count on it. Note: Sight is designed to work on same plane rail systems only. Will not work with a railed gasblock that is higher or lower than the receiver.