The Holosun HS503GU is a red dot sight that features the circle dot reticle. G signifies that the battery is in a round, screw-on turret on the outside of the optic. 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.