Tom... re: Holosun... I have three, two 510, and a 507... the 510 has a jumbo tron view through the lens, and for a competition gun, allows me to track the steel much better... both models have an integral solar cell, the small 507 is on the bridge protecting the lens, and the 510 on the flat behind the lens... they have automatic brightening as the ambient light increases so does the dot brightness...the 1632/2032 cells are not rechargable and the solar cells power the LED with available light..., they both have a "shake awake" function that instantly turns them on...the 510 has a usable circle, circle dot, and dot reticle option... the small one also, but the diameter of the circle is too small for my use... price wise they were in the $310 range, about $75 more than the Vortex, and $150 less than the Trijicon