The HS503CU is a new micro red dot optic equipped with a 65 MOA circle dot reticle with the option of switching to a 2MOA dot only. Holosun's highly efficient light emitting diodes and solar panels allow Holosun solar optics to run without a battery in many lighting conditions. Additionally, the back-up battery lasts up to 5-years on the dot and up to 2 years on the circle dot. The "U" signifies a housing design that shrouds the windage and elevation turrets on two sides.