The circle really helps you to pick up the dot quickly and the sight is really small to help with concealment. The magnetic shut off is my favorite feature.
I mounted this on my SIG M18 Commemorative pistol, with a modified TangoDown plate. Had to use some slightly longer M4 screws to attached due to the TD plate, but worked perfectly! Will have to replace stock front sight with slightly taller one in order to "co-witness," but that is a minor issue!
Nice reflex red dot sight. Window a little small for my taste. The glass is top notch, holds position. I sighted it and now 3000 rounds later still spot on. Was so impressed bought another one.