Both eyes open for scope or open sights. I find that I half-squint my left eye when shooting open sights at long range, tho. Of course you have to make sure the eye you are sighting with is your dominant eye, but you've probably already done that. With both eyes open you've got your peripheral vision and depth perception going for you. Shooting one-eyed you lose both of those.
Some people have a hard time quickly acquiring their target thru a scope, or getting all the components of open or peep sights aligned with each other and with the target. The wrong way to do it is to close one eye, shoulder the rifle and look thru the scope (or along the sights) and then try to track the target down.
The way I learned to do it is like this:
- With both eyes open, and looking at the target, raise the rifle and point it at the target like a stick. Look over the top of the scope (or sights) while you do this. Don't look thru the scope or try to align the sights yet. Just point the rifle like a stick. It will be a couple of inches away from your shoulder at this point.
- Smoothly bring the rifle to your shoulder and lower your head until you've got your cheek welded to that sweet spot where the eye relief and eye-to- scope (or sight) alignment are perfect. Both eyes are still open. This is the part that takes practice, but it's the most important part.
- The target will be somewhere in the scope, or with open sights, they will be somewhere on the target. Make a minor adjustment to bring the crosshair, or sights, onto the spot you want to hit, and that's all there is to it. With either a scope or open or peep sights the trick is to train your eyes and body to weld the rifle to your cheek and shoulder in such a way that the sights are lined up with your eyes properly every time. The sights or scope may not be perfectly aimed at the target, but your eyes, and the sights, are properly aligned every time. Then, a quick minor adjustment is all that is needed to get on-target. I think acheiving this sight (or scope) alignment automatically, and consistently, is the same whether you are shooting a rifle with any kind of sights, or a shotgun.
This takes some practice, but it's not hard to master.