HS515BU is a red dot sight that features the circle dot reticle with the option of switching to a 2MOA dot only. B signifies that the battery is in an internal tray, accessed through the side of the optic with a removable tray. U signifies a housing design that shrouds the windage and elevation turrets on two sides. Employing new LED technology, the life time for one CR2032 battery can reach 2 years if left on constantly. However, the motion based automatic on and off feature extends the battery life dramatically beyond that for the average shooter. Includes flip-up lens covers, a kill flash filter and a quick release mount.