Calibrating the gun requires a means of fixing it in place and aiming it perfectly at the target, ideally from the same distance that you will be shooting from. I used a vise to hold the gun in place and kept nudging it very gently to align the sights with the target.

Once this setup is completed I could find where in the picture the target center is with respect to the 4 IR LEDs and apply any necessary offsets in order to obtain a perfect shot.