On most all peep sights( save those on military firearms) you will find an aperature, or ' eye piece" that screws into a bar that runs across the top of the action.
In the case of the Tang Peep sight, there is a post standing up, that normally can be lowered or raised, when you want to use the sight. The eye piece usually has a 1/4 x 28 threaded section that screws into the post.
How big in diameter the eye piece is, depends on what kind of Peep Sight it is. The modern Lyman and Redfield peep sights, made for mounting on bolt action, lever action, pump, and semi-auto action rifles, and even some single shot, break open guns, have screws to mount the sight base to the side of the action, and then screw adjustments to raise and lower the sight for elevation, and separate adjustments on the bar to allow for windage. The peep used for sporting guns tends to have a small diameter eye piece, often only 3/8 " across, while the aperature on tang sights can run from 1/4" to 1" across. The aperature on Olympic target guns will often be twice that large.
The hole in the middle tends to be about 1/16 inch, or smaller. Sometimes they are too small to let enough light in to see your front sight and the target clearly, especially in early morning, or late afternoon light. You can either drill out the hole in the aperature to enlarge it, letting in more light, or use the small holed aperature when you are out on the range in the bright sun, zeroing the sights in for a particular load at a given range, and simply screwing out the aperature, saving it, of course, and using that large 1/4" hole in the bar as your peep sight.
This large hole has the appearance of a " Ghost Ring " sight, now being made and used by people who were not raised on and trained to use a peep sight. Brownell's carries all sizes of aperatures for the discerning shooter, who doesn't want to tamper with the factory made sight. I have never had that problem. It was theirs until I bought it. Now, its mine! I drilled out my first peep sight, until I could see through the damn thing! When I took it hunting, and it was overcast, gray, and dark in the woods, I took the aperature out, and shot the gun just fine using that large hole where the aperature should be screwed. When I cleaned the gun to transport home, I put the aperature back into the sight, and it is there today.
My eyes get worse as I get older, and I may have to drill that hole out some more to be able to see my front sight on the range, and I will do it when I take the gun out again. The size of the hole does not control the accuracy, unless you are in a paper shooting contest at long ranges, say, over 200 yds. For hunting, where the ranes are usually far shorter than the distance we shoot at a range, the eye will naturally center the peep site as you look through it to see the front sight, regardless of the size of the hole. If you have sighted the gun to shoot dead on where the front sight is on the target, than use that same hold with the " Ghost ring " set up. IF you sight the gun to shoot higher, so you have to hold at 6 o'clock on a bullseye, just hold the same on the deer or whatever the animal you hunt may be.
