Got the 77 out at Redfearns today. Zero was miles off: 2" at 35yds again. Stock screws were all tight. Then for half a dozen shots it was bang on, then off again...
So, I decided to try the RobF front bolts out trick. Zero immediately dropped 2.5" low and an inch right, but seemed to stay there, so I decided to re-zero. It seemed to stay bang on so I checked some of the other ranges which seemed right too. It stayed there for the rest of the session, about 150 pellets until I ran out of what express was left in that tin. I don't like having no bolts in the front. It looks wrong with the holes there, and it feels wrong, but it stacks pellets! I'll leave it like that for a while to see if it holds zero.
Something weird going on with custom stocks definitely.
I then got the 97K out for a plink (GinB Teak stock), with all bolts properly done up, and it goes and drops half a dozen pellets on top of each other and was generally just bang on.