Fpoole - I was thinking about this actually. It would definitely achieve what I'm trying to do. But actually I've ended up ordering a gamo 25mm seal, which fills the void in as the seal doesn't have a hole all the way through it.
Russ - Very neat idea about storing with pressure taken off the breech seals. Sometimes the simple solutions like that escape me
I'm reluctant to go down the route of a pellet probe, mainly because I'm quite likely to lose it/forget it/drop it in the mud.
In the end I think I've done the sensible thing. The 80mm stroke piston has come out, and the 84mm stroke piston has gone in. The extra stroke and less pre-load seem to work OK in this setup and didn't give me any concerns at today's shoot. (Tony can feel free to say 'I told you so' here). This meant I could use the new breech seal and my favourite pellets, without any lead swarf being created which was the entire point of this exercise to begin with.
It didn't feel quite as soft as the lost volume test, but certainly not bad. More than good enough to knock tin chickens over if I do my bit right.
The 80mm piston is now waiting for the new seal I've ordered, and it will be going into my SPARE gun like it should have done from the start! I also have a spare comp tube and end plug for that. I think I'm going to fit a new 3mm transfer port. Then open it up slowly, measuring efficiency and trying to judge shot cycle. I guess in stages of 0.1 or 0.2mm. This will have to wait until my friend with a lathe has finished moving house because I don't have the tools to be this accurate.
I'm an impatient person, and find it hard to let an idea go once it's in my head. I just have to try it!