Why It's There
Our eyes can only focus at one distance at a time. Rifle and pistol shooters know that to hit with iron sights, they have to let the rear sight and the target blur while they keep the front sight in sharp focus. Try that with a shotgun-as many people do-and you'll miss. Here's why "drawing a bead" on a target is the worst way to shoot a shotgun: As long as you're looking at the target, your eyes guide your hands, which in turn direct the barrel to the right place. As soon as you glance back at the barrel to measure your lead or check your "aim," your eyes stop feeding your hands the data needed, and the gun stops dead.
To hit with a shotgun, you must keep your eye on the target while the barrel of the gun registers as a blurred smudge in your peripheral vision. That's where the bead comes in. Don't think of it as a sight; it's more like the red handkerchief you tie on a long 2x4 sticking out of the bed of your truck. It serves as a reference, as a flag or marker, so you're aware of the muzzle as you bear down on the target. You should never carefully measure leads, but the front bead can help you see in an instant when you've got the right gap between barrel and bird.
Don't remember where I got this, but it's the truth, and I only share the truth.
