How do you zero a red dot sight on Crosman 2240 co2 air pistol?

Zeroing a red dot is the same as zeroing a scope. Place a target about 30 feet in front of you. Use a table or post to steady yourself. Aim at the target center and fire 5 shots. If you are striking the target (Lets say to the left of center and high) then adjust your Left right knob on the sight a few clicks to the right and fire 5 more shots. do this until you are on a center line with the target. Then do the same for the up / down adjustment knob. Do this until you are reasonably close to center. I highly suggest you use very good pellets. Do not use lightweight pellets. Use "Match grade" pellets. Crosman makes them.

Always hold the pistol the same way. How you hold it effects each shot. I use a sand bag to steady my air runs