Nice to be reading your posts again.
There seems to be a number of issues here ( which all helps to confuse things ).
I'm convinced that my problem with the main 77k is related to temp and variation in piston seal fit. That's whether the temp is outside temp or induced via slow or fast mode. I've been saying that all along. Hence why I think Jim's theory applies to my problem. So maybe an O ring is the way to go with that one.
I'm also going to remove all the lube around the seal and replace with dry. Matt advised me of that and I've also read Tony's comments re that on other sites.
The JSB comments interest me.
Like most I had a favourite batch of JSB ( Mine were actually AA Field ). Used them for ages a few years back and the gun always seemed on zero. I've racked my brain trying to find anotherJSB batch that suits the gun like those did including the infamous batch 18 AA which were basically dogsh1te.
The old ones used to go over the chrono with a very tight spread. Recently ( best part of a couple of years ) I've been getting, at best, 20fps spread out of the tin. I never really worried too much about that as I was still getting excellent groups at 35 yards out of the tin. Even when the shift happened they grouped well ... just 20 - 25mm away.
I've kept saying that the jump in MV from average to average ( hot to cold ) could be 40fps ( this is with the newer 20fps pread pellets ).
Now using this new 77k ( Bigtoe design tune ) I tried Prems out of the box and found them very accurate at 35 yards ( and it seems easy to shoot good groups ), so I didn't bother doing much shooting with the AA/JSB. I just used the Prems. They are tighter over the chrono. More like under 10fps spread. I Bic'd the Prems and found there to be different sizes in those. They are all enormous. You have to load them in with a toffee hammer. This is all a year or so ago before the knee exploded. The plan was to buy some Prems and a sizer. Then size them all down to the same size and keep trying until I got a size that the Bigtoe 77 liked best. Then I could just size all the Prems I used to that size.
I just got so fed up with JSB/AA and spending a fortune trying to find a decent batch.
It would be interesting to see if the sized Prems keep the spread down over the chrono ... and group well ... and don't shift POI as much.
( Oh I had an old tin of Mozzies given to me a while back from Frank at Rivi. They were brilliant. They grouped well right out to 55 yards ... but a tin doesn't last forever ).