Again, I advocate training yourself to shoot with both eyes open. Aside from the others reasons I cited in the other thread, I'll give another. Although, this has less affect on defensive shooting and more on long range or prolonged shooting, including sniping.
Long range shooting in the case of the sniper, bench rest shooter, service rifle shooter, etc. spend a fair amount of time behind the sights either observing or waiting for their shot. In the case of the sniper, he and his spotter spend time behind magnifide glass waiting for the target to appear or gathering intel. The bench rest shooter is doing much the same thing as is the service rifle competitor during the 20 minute slow fire, albeit without glass.
What all three of these individuals have in common is that they are trying to deliver the most precise and accurate round down range. In order to do this a lot of things must occur and be taken into consideration. In other words, your mind must be continuing to process incoming data constantly before, during and after the shot breaks.
A brief overview of the of main mechanics to marksmanship is - Breath, Relax, Aim, Slack, Squeeze. A lot of attention is given to breath control, trigger control, sight picture / sight alignment. However, I think the relax phase is often overlooked. A natural point of aim is required to allow the shooter to relax natural support and not "muscle" the firearm on to the target. This means as much contact with Mother Earth as well as bone on bone support. It also means breaking the shot during the natural respiratory phase, relaxing the support arm and grip.
Everyone knows that holding your breath too long, or keeping muscles tense causes the entire body to tense and begin to tremble. There is another action that will cause the body to become tense and tremble. And, that, my friends, is holding one eye closed for any length of time. The face begins to twitch from the extended contraction of facial muscles needed to close one eye, and it quickly spreads to tensing other muscles.
Now, back to the need to process information. Even though you are concentrating with dominant eye, your mind will still process information received by the non-dominant eye. This means detecting any change in wind speed or direction, detecting a new threat, etc. Another phenomenon that occurs is blurred vision for two primary reason.
One is the case of one eye looking through magnifide glass, and the other is the fact the closed eye's pupil will dilate because of less light. The magnification problem is greatly reduced by keeping both eyes open and the dilation problem is eliminated.
You can see (no pun intended) that there are many benefits of keeping your eyes wide open and really no benefits of closing one eye. People who say they can't shoot with both eyes open have never trained themselves to do this. It takes conscious training and pratice like any other aspect of marksmanship.
