TargetTalk

Whenever I shoot diopter/peep I always have this issue.
The hole I see through the rear sight (diopter) should be round but isnt.
On the underside its flat.
What causes this?
Eye to sight distance? Diopter height?

It may be diopter height. Maybe you see inner edges. Things to reconsider are eye relief, cheek piece height or maybe whole device, if it's not on its extreme settings.

From memory, a flattened bottom of the aperture means the distortion is actually at the top. Is your head tilted heavily forwards? Are you peeking out from under your eyebrows?

If it was the cheek getting in the way, the distortion would show at the top of the aperture.

Tim is correct, a flat at the bottom of the rear sight aperture is a sign of the comb being too high.
Cheers
Martin

I was going to say the same as Tim and Martin - I would guess that your head is tilted downwards and your eyes are tilted up and the skin under the eyebrow is occluding your view of the iris. This happens a lot if you are closing your non-sighting eye rather than using a blinder. Having to tilt your eyeballs up to see through the sight is very tiring and I found it leads to involuntary movement of the eye which is definitely detrimental to a good aim.
Solutions are the lower the cheekpiece (if your stock allows) or raise the sightline using raising blocks which is my preferred option as I find it results in a more relaxed neck and upright head with the chin forward.

It's also worth checking your eye relief - if you're really close to the iris, then the peephole may become so apparently "large" that you see the edges of any anti-glare tubes attached to the front of the sight rather than the edges of the iris.

K.

I noticed the same last time I shot prone since it was first time shooting smallbore in a couple months. Didn't know this was a common thing, thanks for the info.

Thanks all for the tip!
I´ll check my check piece height. Fortunately, it is adjustable.
You may be right about me having my head forward.
Diopter is on the action rail at the mo, no risers.