My BM AR carbine has the standard A2 rear sight.

When I first got it, I calibrated the sight for the *improved battle sight zero* which IIRC is a 50/200 yard zero correct?  And when you zero the rifle, you loosen the tiny 'lil allen screw down inside the sight and turn it *back* two clicks, so that it's 2 clicks before the 300 yard mark.  Well, the last few outings, the damn thing will jump forward 1 click from the bottom in 1 shot.  I can turn it back every shot and it just keeps advancing that one click.  It's annoying the hell out of me.  It's fine for 2 years and suddenly it moves on me?  

Any ideas-suggestions-comments?

Thanks

I could not tell you. I have NEVER loosend or even touched ANY kind of allen screw on my sights...

Try loosening the windage wheel and set the sight back one more click (giving you a bottom setting of 8/3 -3).

Sometimes what feels like two clicks is really 1.5 - haven't had recoil adjust my sights, but that extra click should free up some extra room for a solid 8/3 -2 setting.