Re: 590 special purpose
For me the primary purpose of the bead is to check alignment between the bead, and the top of the receiver. I don't really aim with it. I snap the gun up, and if the bead is riding at the right height relative to the receiver (and the only way to know what that is for your eye, and your gun is to do something along the lines of nitesite's suggestion) , I know the barrel is in the proper vertical plane. I then "point" the gun. I don't "close one eye and take a "bead" on a target" unless I'm shooting slugs at greater than 40 yards. I "see" the bead but my sight picture isn't Clear bead, fuzzy target. It's really fuzzy receiver, slightly less fuzzy, but still pretty damn fuzzy bead, and crystal clear target. But I'm an old wingshooter. Out at 50 or more yards, I do revert to shooting my shotgun like a rifle. Clear bead, fuzzy target.
Edited to add: I think a lot of folks buy shotguns as their first guns for SD and don't understand how to shoot a shotgun. Back when I was learning every old timer I came across had the same message "that's a shotgun, not a rifle son. Shoot it like one!" Obviously ghost rings and all that can change the equation, but going to the trap range and learning to shoot it like shotgun is probably the best way to make peace with the bead.