Help with front iris and rear sights

[usmcmba]

Hi list.

I am relatively new to shooting with iris such as the Gehmann etc and I am going nuts with groups shifting (or walking) where I would have a group right in the center, but all of a sudden the group will shift (eg from the 10 ring then to the 9 or 8 ring in the 9 o clock etc). What is really frustrating is I have a nice group, but all of a sudden they will end up outside of the black, while if they were in the center, I probably would have had a near perfect score. Also, last time I was out at the shooting range (a very bright sunny day), I noticed that when the group shifted and I tried to compensate by trying to make the correct adjustment on the rear sight, nothing happends (such as making 10 click to right and instead of the next shot going 2 1/2" to right, it doesn't move at all, like no adjustments were made)

It seems I am only having this problem with a match rifle that has the fancy gehmann iris sights etc, while standard iron sight on service rifle this issue is not a problem for me. Based on this, I can only narrow the error down to what appears to be improper use (or lack of knowledge of )of sight system.

My current setup involves a 22 mm front iris (Gehmann 522C) with a 1.5 magnifier and I usually set the iris for 4.0 or so for the sitting and kneeling and 4.8 (max) for standing. In the rear I have a centra match sight with a standard gehmann iris (no color, diopter etc). It seems most people who have the most problem with these sights tend to get one of the rear apatures that have a lot of glass such as the diopter, color filters etc for high power shooting. Also I want to mention that I have an 6" extension tube on a 26" barrel.

I suppose my question is, is it really true that the more glass you have in front and rear sight, the more likely a group may shift? Someone told me also that a 1.5 magnifier in the front sight has a tendency to change the POI quite a bit, is it correct? Also, what happens if one were to to set the rear iris correctly (such as allowing too much light or not enough in?). Does a bright day have a tendeny to change the POI etc?

Thank you in advance for any thoughts on this.

[Shooting Kiwi]

Sounds much more like a mechanical problem in iris mechanism mounting, sight mounting or adjustment mechanism than an operator error. Something's loose, surely.

[keithwwalker]

I think what you are alluding to is that your rear sight or your front sight is somewhat misaligned relative to each other or the rifle.

Even flat glass has a diffraction effect and may cause the shifting that you are experiencing if the lenses are not perfectly perpendicular.

I have noticed that some iris elements have a spirit level:



Unfortunately, it seems they are only designed to ensure that your sight is level within your right/left plain when you are aiming.

It would interesting to see if you could adjust your sight with the spirit level with the rifle in the vertical up/down plane, that way you could see whether your sights are perpendicular in the second plane.

Further, it seems that many sights have only one allen key to hold the sight to the dovetails. Depending on the dovetail(s), there could be a misalignment that may have to be shimmed, but only if you have a loose sloppy fit.

I just started researching iris and sights, so perhaps there is someone more experienced can chime in, I am speculating.

[Barney]

Most rear sight adjustable irises come with a locking ring which you can use to adjust the postion indicator mark on it so its easliy seen while in postion, ie 12 o'clock, 3 o'clock etc, once it has been tightly screwed into the rear sight. It may be this ring which is binding on the sight body causing the rear sight to lock up and jump instead of moving freely.

[artandscience]

I hesitate to suggest this because it seems so obvious, but..

Realize that with diopters you need to have all three (not just the front two) rings centered visually. The rear diopter, as well as the front ones. It's quite possible to have the front sight perfectly aligned (equal white around the bull) and be way off to one side from shot to shot if your head position moves and the view through the rear peep doesn't center the front sight.

Just a thought.

I never found this to be a problem in smallbore, but in fullbore my eye sits some distance from the rear peep and I found it much easier to misalign things.

cheers,
stefan

[adds033]

I hesitate to suggest this because it seems so obvious, but..

Realize that with diopters you need to have all three (not just the front two) rings centered visually. The rear diopter, as well as the front ones. It's quite possible to have the front sight perfectly aligned (equal white around the bull) and be way off to one side from shot to shot if your head position moves and the view through the rear peep doesn't center the front sight.

Just a thought.

I never found this to be a problem in smallbore, but in fullbore my eye sits some distance from the rear peep and I found it much easier to misalign things.

cheers,
stefan

X2 artandscience, i am with you i have shot both small/fullbore and agree totallly! I must say that i naturally try and centre the front sight thru the rear peep! seems to help my group!

[jhmartin]

Are you using an occluder?

If you make your own and it sits on the front side of the rear sight diopter/iris system, it may be binding the sight a bit and causing some hysteresis that looks like backlash ....
symptoms of this are good groups but the impacts don't change with small adjustments and then all of a sudden jump.