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. - Automatic on and off based on motion - 2 MOA dot & 65 MOA Circle - Reduced training time and ammunition - Unlimited field of view - Parallax-free with unlimited eye relief - Durable and reliable construction - Low mount and 1/3 Co-witness mount included