As a firearms instructor for 20 plus years I can give you a few things to consider.
Handguns only:
1. If you are going to the range and shooting at a target with one eye closed you are learning to shoot accurate at a target. You will become a good target shooter or hunter.
2. If you want to learn shooting in a combat situation you have to practice with both eyes open, so when you point and shoot your dominate eye puts you on target without having to use you sights.
3. You have to practice as you would in a shooting situation. Short range, quick point and shoot, double tap every time, shoot no shoot situations, moveable targets, and while moving.
4. You have to practice reloading drills, and misfire clearing drills as much as you practice shooting.
5. It takes about 2000 reps of anything for you body to start to become natural at that function.
6. Looking for cover and moving is sometimes more important than shooting.