Here is another theory, based on experiments i have conducted with 23 22 and 21mm pistons.
All guns were set to produce the same ME, however due to less air being compressed JSB batch became incredibly important, my TX moved from shooting JSB's the best to shooting superfields much better. So less air changed the characteristics of the guns with previous favourite pellets....so I got thinking.
On a standard stroke 25mm 77k you have around 36cc being compressed, note I say 36cc not 40, you lose around 4CC until the seal has enough pressure on it to seal correctly. depending on the density of the air this 36Cc value will go up and come down.
I have been testing small piston set ups, with a 22mm HW seal the shot cycle is soft, however I was able to induce POI and velocity shifts by shooting the rifle very quickly and heating up the internals. I also tested the rifle kept cool and then warmed in the sun, again the velocity went up and down, so I bagged the seal and went back to an O ring.
So why an O ring?
Ed canoles in the US shoots US HFT, he shoots in the mountains, he can see snow at 6am and 70f at lunchtime, he was getting big POI shifts so he converted his 95 to run an O ring nose...POI still there BUT massively reduced, he also has tried dry powder lube and krytox etc....least lube the better.
I am running a bronze piston nose, flow enhanced on the face, minimal o ring crush, and to date have not seen much of a velocity change, the 440Th is the best out the bunch with a 22mm piston which looks to be the most consistent also. Bronze was chosen because it has an expansion rate very close to steel...as they action expands or contracts so does the piston I hope at a similar rate. The o rings seal at close on 1mm of movement, maybe less...efficiency is what I was looking for.
So what can be done....we need a way of metering the pressure in the compression tubes, so that the pressure remains close to the same regardless of pellet weight and fit, density of air and temperature. An accurate pressure relief valve if you will. If we get a consistent amount of air being compressed it should take one variable out of the equation.
Last...back to the JSB's, with the small piston rifles, especially my 77k, i noted that pellet head size, skirt size and weight had to be very consistent...my 2013 die 34's are awesome, my 2015 pellets are utter shyte in all but 1 of my springers...and thats my LGU....which is still a 25mm set up but also has a tighter barrel.
You gents seeing POI shifts, are you all shooting Exacts?