The only time where I close an eye is when I am well hidden and try to take a very long shot, near the maximum effective distance of my pistol. Then I can relax, breath, take a few seconds to align my sights perfectly and fire. Otherwise it is both eyes open and focusing exclusively on the target.
With time I found that it's easier to rely on my muscle memory as Mitchell12 said. It's all in the wrist as some would say, and the more I use my pistols, the more I can rely on my wrist to align my sights correctly, so I don't have to aim as much as before, or at least, spend a lot less time aiming and taking more reflex shots that actually hit, even at range.
Keep quiet. Sound travels faster than BBs.
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