Let’s try a drill. Hold your thumb out in front of you and focus your vision on it. Alternate shutting your eyes. As you swap eyes you will perceive a movement in perspective. In other words, it looks like your thumb has shifted left or right. The reason for that is your eyes are intended to work together, triangulating vision between the three points of left eye, right eye. and object of focus. So when one eye is closed perception is skewed. A shooter could compensate for that error by adjusting sights but if that shooter were to fire with both eyes open the shots would be off the mark even if everything else were properly executed. The problems with keeping one eye closed:

  1. Spontaneous defensive shooting is likely to include both eyes for which the shooter is untrained.
  2. Closing one eye is a matter of degrees in relation to squinting with the open eye. When the muscles around the shooting eye are flexed differently each session (if not each shot) the results will naturally vary.
  3. One eye closed means one half of your world is blind and you are taking a risk that nothing will pass in front of your weapon from the blind side or present a threat without your knowledge. That’s why boxers get vision tests.
  4. Most importantly, above all else, a one-eyed shooter, even after adjusting the sights, is still making sight alignment and sight picture decisions based on one visual survey rather than two. The shooting eye has no verification.

People with vision in one eye can shoot well but they must work all the harder and they have the advantage of not having to close an eyelid and contract facial muscles which may influence the shooting eye. If you have two eyes, use them.