Rear sight aperture "bowl" on AR-15

[MMcfpd]

I could conjure up some explanation - probably having something to do with gathering light I suppose, but I'd rather see if anybody does really know.

What is the purpose of the "bowl" surrounding the aperture on most AR rear sights?
http://i150.photobucket.com/albums/s95/MMcfpd/A1_sight.jpg

I've seen them mounted both facing the shooter and opposite. That makes me think there are quite a few people that don't know its purpose.

[Slamfire]

Rear aperature holes have sharp edges. I assume the countersink is to reduce reflected light.

[olyinaz]

Less area inside the hole to fill with crud as well.

Oly

[444]

That "bowl" is perfect to fill up with water when you are shooting in the rain.


I realize that it is vertical when you are firing, doesn't matter. A big drop of water hits there and stays there.

[Casimer]

I think that it's primarily to reduce the length of the peep hole. So your sight image is less likely to be affected by the angle of your eye.