The length of the hole, if excessive, causes glare from the sides of the hole, due to frontal light reflecting off the sides.
If the hole is short enough, there will be reduced glare.
A sharply tapered hole (from the front) that leaves a knife edge hole at the rear, may be cocked slightly to the front sight and still give a perfectly round sight picture.
The longer the parallel sides of the aperture hole, the more perfectly in line it must be, to the front sight.
To reduce glare on a drilled hole that is parallel, placing a sight shade over it will eliminate that glare.
This is a picture of, Roger Fisher's chunk rifle (for our postal matches) with sight shades.
A smaller sight shade can be made from the old black 35mm film containers, or even shotshell cases.
Simply split and they will fit over and shade sights with dovetails wider than the top flat.