TargetTalk

Thanks to the much helpful advice I've received here, things were going fairly well over the autumn, but now I'm going through a bad patch again! (So's everyone else in my club:) Current most annoying issue is massive vertical changes in PoI, both up and down - even into the 8 ring sometimes. And I don't call them.

Haven't changed much: I wear a lens on a head band, and I've moved it a bit further away from my eye, but I'm trying hard to keep it centred. I've also switched ammo from RWS Special Match to cheaper Rifle Match, but it's totally implausible that a batch of Rifle Match could be so dud as to have anything to do with it.

The other change is that I've moved from the downstairs firing point (where I often get too cold for comfort) to the upstairs point, where I can get too warm for comfort! Does being too warm exacerbate pulse problems, perhaps? I do sometimes get those..

Julian,

This does seem to be a problem for you. In terms of position I'd look at:

Sling length - is it the right length to hold the butt firmly against your shoulder? Does it stretch in use?
Sling fit - is the sling pressing on the arteries in your arm, and picking up a pulse?
Cheekpiece - is this fully supporting your head, and guiding your eye behind the sights (horizontally and vertically)?
Eye Relief -is the rearsight too near/far away so that you're cramping or straining your neck to aim?
Butt plate - does the height bring the sights comfortably onto the target, and at a comfortable height for your shoulder/neck? Does any offset bring the sights to your eye: not too far either way? Does the hook locate the plate, without catching?
Trigger - can you locate your finger consistently?
Jacket - does the right (sling) shoulder stay put -movement can let the sling drop. Is the left shoulder pad really smooth under the butt?

Normally I'd think the shooter's head is moving, the rifle is moving up or down, or their breathing and/or NPA technique is off kilter. If the shifts only occur on a card, and your group on the sighter is consistently tight, look at how you navigate each bull. Be honest, because if your group on the sighter is strung, there's a deeper issue. Make sure that you are properly sighted in first; I've seen many shooters correct off the edge of their group that's nearest the bull (and under correct too), and wonder why their group is still off centre.

Does the rifle string shots vertically even from a benchrest? If it does, you might want to check the bedding of the rifle. If it shoots nice round (or reasonably round) groups, the fault is probably with you or your position.