Rear Iris - Help !!

[Fimbulvetr]

I decided to swap the rear iris on my BSA Martini. The Gehmann 575 looked like a good bet, 30 year guarantee and all bells and whistles present, so should never need another. I saw (through Intershoot website) that 2 versions were available, one 'normal' i.e Anschutz thread, and another for Parker Hale i.e with adapter for PH thread. I am using a PH25B rear sight.

I already have a PH->Anschutz adapter (never been used) so I bought just the normal 575 with Anschutz thread.

Desired effect was not obtained when I screwed it all on !

1. The sight picture has changed in that the rear sight now appears to be over (above) the front sight, I'm now looking over the top of the front sight element.

2. Rotating the rear iris shows that it doesn't appear to be central, i.e the sight picture moves around in a non-concentric orbit, almost egg shaped (mildly).

I don't understand why swapping the iris makes the rear sight appear higher (too high) - I took off the PH iris, which I was looking through the centre of, and swapped it for the Gehmann, which I was looking through the centre of. How is it now higher ?

Also, why does it appear to rotate around a non concentric orbit ?

It would be easy to say - the PH/Anschutz adapter is faulty. BUT, by coincidence, another club member was present trying to do exactly the same thing with his BSA Martini (what were the chances of that!).

He had a PH/Anschutz Adapter (different model to mine, brass and slightly longer) and a Gehmann rear iris (less complex than the 575). He suffered exactly the same problem ! Sight picture is now over the top of the front element and not a true circular rotation should you feel the need to rotate it (by this I mean rotating the adapter when the iris is attached, ie tighten/loosen it).

We screwed a simple Anschutz iris onto both thread adapters, the foresight was perfectly central on both guns, and iris rotated around the proper centre point - no problems. So the thread adapters are fine.

I put my 575 on an Anschutz sight and it looked perfect.

Can anyone explain what's going on ? Wondering if I might have to give Stephen Hawking a bell.

Any suiggestions gratefully received, as long as it's not an offer of fifty quid for my 575.

Thanks!

[andrew375]

You will probably find that the face of the rear aperture hanger is not perpendicular with the bore, also the thread may not be square with seating face. This problem is amplified by the extra distance the light has to travel through the adapter and eyepiece.

My advise is to ditch the idea of using an adapter altogether as, by necessity, the bore of the hole so small it seriously restricts the light entering the sight and so negates any perceived advantages brought about by the more modern eyepiece. There is a reason why modern rear sights use the much larger 3/8" thread. Bear in mind that our athletes brought home plenty of Olympic medals using BSA internationals equipped with the same basic eye piece you have.

[BigAl]

I have used the Gehman Iris with IIRC 5 coulerd filters fitted, which was generally used on my Anschutz 6570 (I think, the all steel version anyway) on my Anschutz 1813. I have also used it with the thread adaptor on the AJP TZ rear sight on my Fulton Mauser 7.62 TR. I had no problems at all with fitting it to the rifle, apart from the fact that it dose move things back a bit close to the eye when shooting a CF rifle.

Alan