Right handed, left eye is the master eye, and I shoot IPSC!
If I try to shoot with both eyes open with iron sights, I can't tell which image is the true sight/target alignment, so I shut my right eye.
Of course, if I'm within 'hoser' distance, I'm just shooting over the barrel of the pistol anyway, and after 20 years practice I manage to keep the hits close to the A-zone.
Funny thing, a year ago January I discovered that my eyes had deteriorated so that I couldn't see a clear sight picture with either eye. I'm far-sighted, increasingly so.
So I bit the bullet (sorry) and got an Open gun. C-more on an STI, bright red dot, no need for sight alignment all I need to do is put the fuzzy dot image over the much-clearer brown target and assume that this is the sight-picture. No problem, right?
Wrong.
I can't keep the right eye open, 'cause I still see a massive blur instead a a darn dot! I tried for several months to train myself to use both eyes, but I still have double-vision (essentially) at 3 feet ... length of my arms ... and I'm apparently too old to train my brain to get past this.
It's probably more a psychological thing than a physical (eyes) thing, but I got tired of fighting for target acquisition. IPSC is a game of seconds, and I can't afford the time during a match to stop shooting and look for target, then sights, then target, then sights (wash/rinse/repeat).
And before you ask; yes, corrective lenses would fix this, but it's impossible to shoot a run&gun stage with reading glasses which turn the targets into brown blurs against a brown berm (same for props, rocks on the ground, etc.) and bifocals give me a headache when I'm switching between image fields.
Jerry the (Do I sound defensive?) Geek