When I've seen 77/97s shift its always been in the vertical plane. I've had it happen on my rifles running vmach seals, and when I've had front stock bolts in.
I now leave them out, except my 25mm in a synthetic stock which doesnt do it, perhaps because I've made the holes bigger.
The vmach seals seem to give a shift depending on temperature. Power did change, but only about 20fps, not enough to see an inch poi shift at 25 yards based purely on power. I think its the pellet exiting at a different point in the firing cycle.
Perhaps there are other causes, but I'm convinced (and have tested to my satisfaction) about these two.
Only problem is if you just use the large rear bolt is it coming loose, as happened for me this year at the Fort double header. Didnt threadlock it did I?