Well this is really doing my crust in now. In this thread there's mention of 20 clicks for me its 17 (Nikko3 on TX). That throws up one of several things I just can't figure - ie why when it moves is it exactly the same amount every time?
In my case its definitely to do with the bolt that secures the stock at the back. I had been flapping trying to get my 77 in its lovely new stock to give me a working gun and time to work on the TX, only to find that the 77 behaves almost exactly the same as the TX. On both the zero can be moved up/down by 1.5" at 45 yard by loosing or tightening the bolt 1/4 of a turn either side of 'nicely nipped up'. The startling thing is how similar the effect on the 77 is; considering its a different construction (threaded trigger block) different stock, different wood (laminate) and a really snug fit (unlike the Ginb).
So what is happening? Is the quarter of a turn bending the action? Can't see any other explanation that would be so repeatable and it is repeatable. After a shift I can get usually get back on zero with one or two tweaks, quicker than moving the turret on the 77 as I can get to the bolt without taking the hamster off. Its as if the amount of turn isn't that critical, somewhere between 1/8 and 1/4, there's a sort of wide (ish) sweet spot where its sits back on the zero.
If the tension on the bolt is deflecting the main tube that the scope is mounted to, I'd prefer to have the action loose in the stock so the action isn't stressed but that's not really practical with a springer. NB As far as I can tell the front bolts seem to make no significant difference, its even tried just having one in!
Even more confounding is what causes the sudden shift while the bolt (as best as I can tell with a dab of tipex hasn't moved) - it'll move over 8- 15 pellets, the 77 moves quicker. I don't have a torque wrench but I have tried loose, quite loose, tight, and almost murdered and the shift will still occur. I have also tried various arrangements of washers and split washers to tension the bolt, so far to no repeatable benefit (though I'm hoping I have the 77 stable enough to get me through 40 shots without a shift come the weekend).
Another thing I have tried is changing from Sportsmatch to BKL's. My thinking behind this is that the front and rear scope mounts sit either side of where the bolt pulls into the action. As Paul L pointed out if the tube is deflecting the stress on the scope body is changing too, my guess is this is something to do with it being the same amount of shift every time, especially as the higher mounts will amplify any bend that occurs between them. Next job is to try AJP rails but strong as they are I cant' see them resisting deflection if a steel tube can't.
I have been at this on and off for days now and and although I'm fairly sure I'm close to knowing the cause, I don't feel any closer to a solution. I thought that reinstating the trigger guard and it acting as a tensioner and spreader, might be a good stabilliser but am less convinced having caught up with the thread. Its still on my to-try list though if I can get the Ginb inlet to accommodate it. The only other thing I could think of is a three point metal to metal fixing set up that keeps the action away from the wood.
I would like to agree with you but then we'd both be wrong....