If you are looking for a Red dot sight without breaking your entire bank account, the Red Dot Sight 1x35 is a good option. The red dot has a diameter of 35mm and can be mounted on any standard RIS (20mm - 22mm). The red dot can be set in 2 different colors, red and green. The dot brightness can also be adjusted in 5 steps as desired.
The Red Dot Sight 1x35 can be calibrated both horizontally and vertically. Operates on one CR2032 battery.