I don't know Jims theory.
What I will say is that it seems to be a mechanical shift. I haven't got the best evidence to back this up, but this is what appears to be happening to me.
Like many i get excellent groups, on par with PCP right out to 55 with the 97. Over the chrono, it's good. Having shot through one while it's shifting, the chrono variations don't seem to correspond with the POI shift. On a still day, or indoors, I can litterally draw lines up the paper... indoors at 25yds it moves perhaps a CM or so despite putting them through almost the same hole for a while. Temperature in this time is stable and I can be shooting for hours... and it can either move at the start, mid, or end of session or not at all. When it goes, it creeps up then settles, then can wander again.
Sometimes i've had so much shift that doing my ranges I could hit a pellet mark at 55 on my 45 yd, and then another day the other way around.
What I will say is if i lift and shift the turret onto the new marks, all the ranges come in. I still will have say 5 MOA between 25 & 55 yards, despite having just dialed in 2 MOA of shift. This says to me it's not power... it's mechanical. The pellets are still moving through the air in the same trajectory, it's just the scope is no longer looking at the same place. Barrel and scope are no longer aligned as they once were.
So what can cause that in just the vertical plane?
Underlever catch? We'll mine's standard, but others that have seen it have had Venom/Vmach (not sure which) silencers. That's a different catch method so that rules it out for me...
Stock? Mines had front bolts in only, front bolts out, back bolts out only, back bolts and front bolts in. It's been pillar bedded, and clamped to pillar so that there's no wood touching at the clamp points inside or out at all (the wood sits around them)... that made a difference to feel, but didn't solve the shift. It's now sitting like a PCP on one bolt, and floating off the stock on the pillar. It's been set up using a voltmeter so we know exactly when metal makes contact, and there's no squash pressure...
My pet engineer doesn't like the cocking linkage having no slack. My TX does. His theory, which i can't fault, is that the piston slams home and hits the compression tube, which kicks the underlever linkage all up the chain into the barrel... so if the seal on the piston does vary in temp, it might give more or less kick? But i thought i saw temperature correlation, but now i don't. One thing I remember is that I'm not running out of scope travel... the POI goes up... and then it goes down another day... I need to log it better to confirm.
My theory is the action is just bending at the loading port. But my pet engineer wont weld 6" of steel across the top of it... mutters something about sacrilege or something.
Another thought, again related to the linkage thing, is that something is rotating inside, and giving more or less impact over the firing cycle. But I don't know what, cos i haven't had it apart.
Now i've never changed the scope, but the TX has the same one on... and i'll be putting one on the 97 next to double check... but im pretty certain it's not that. And by testing with unadjustable mounts, adjustable mounts, and adjustable mounts with loctite holding them so well i don't need bolts in, i've ruled them out. I've had rails on and off, so they're ruled out.
Does Vmach make their own compression tubes to enable them to be jewelled or blackened ahead of orders? Do standard 77/97s have any slack where the linkage joins the tube?
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