I solved my zero problem on the 77K. The zero had shifted about 2"-3" up and left at 35 yards, no bloody wonder I didn't hit anything in the last third of the course! Grouping was crap too.
Can anybody answer me this: why on earth does the front stock bolt "yoke" on the 77s move?? I cannot understand what benefit that brings.
Anyway I must have tightened up one side first which pulled the yoke away from the bolt on the other side. Because of the slight misalignment of the CS stock holes, the bolt had entered at a slight angle and started to cross-thread. Luckily due to the washers I was using, I then just didn't have the bolt length to screw it in any further the other week, so limited any damage to the first 2 or 3 threads. No wonder the zero went off along the course, it must have only been held on one side of the stock.
When I put the stock back on this time I did it by engaging just the first thread on the bolts on each side, so there was no pull on the yoke either way, and then tightening up alternate sides, half a turn at a time. The bolts on each side now both seem to be central and tight. After re-zeroing accuracy is spot on.
However for some unknown reason even though it's the same mounts, the same scope, the same zero range, the same pellet batch and the same velocity give or take 3fps... the trajectory has changed! It now seems to be dropping a little bit further at longer range. Bloody springers!
Just had a thought about raised scope rails on FT springers. Couldn't a long alloy rail, clamped along its length to a steel cylinder, act like a bi-metal strip and cause some flex in different temperatures due to the differential expansion of the two metals?