Two more experiences to mention. First, the Huma regulator in my Caiman X is definitely better at avoiding the first shot being lower in velocity. I retuned my Caiman up from 890 to just over 900 fps with H&N 18s and the first shot is no longer low. It was at 890 but is not at 900. The peak is at least 930 so I am still well under the peak but the first shot velocity is fine. I'm liking this tune but today I shot a pretty poor 30 yard challenge target so I may change it again. But probably not, I think it was just bad trigger pulling by me on a few shots.
More complicated experience is with my P35-25. I tuned it over the winter to about 800 fps with JSB MKII "heavies". Tin says 33.95 but my scale says they weigh more like 35 grains. But I took the gun out to shoot it and it was empty. So I changed a couple O-rings and it seemed to be holding air so I shot it. Velocity was under 600 fps! Tuned in cooler weather to 800 and in 90 plus degrees it shoots 600. I was not quite at peak hammer spring so I increased to as far as I can go (and still cock the gun) and got to 615. From previous experience my strong suspicion was that the regulator was too high for the hammer spring so I turned it down. Velocity went up. I decided not to try to live on knife's edge and left the regulator turned down a tad so I can back off the hammer spring a little and still get 765 fps or so. I think I could get to 800 but it would be at very near absolute max hammer spring (again) and I don't want to be loosing 200 fps seasonally and have to change the regulator. It is internal which doesn't make it terribly hard but it tends to lead to the need to change O-rings and refill SCBA tanks too.
But one theory about colder weather is that it decreases hammer spring force by thickening any lubricant in the hammer chamber. That cannot be what happened with my P35-25 when it went down by 200 fps going from ~60 degrees F to ~90 degrees F. I think the energy in the air went up at the higher temperature making the hammer too weak to open the valve. Nothing else fits what happened. I guess it could also be that hot weather increases the regulator setting. I don't see how but that would also explain it. I already found data illustrating that higher temperature air does have more energy but it was not enough to explain the velocity loss in cool weather. But at least around here humidity follows hot weather. It gets knocked down by the dessicant on the compressed air but maybe the relative humidity of the high pressure air is still higher in the summer. More moisture in the air would also raise it's energy. Maybe the two effects are enough. I am not advocating putting more moisture in the air we use, just trying to figure out why airguns loose so much velocity in the winter. I'm pretty convinced that it is lower energy of the air.
Anybody know what nitrogen does with temperature change? Nitrogen cannot transport water so that effect would go away. Air is mostly but not totally nitrogen. I think it's the oxygen that transports the moisture.
. But I'd say the estimation of 10 fps lost or gained for every 10F degrees difference is the correct one. I did not notice much difference when shooting (presumably) regulated PP750, but it's a pistol with a smaller air tube. Will have to double-check later...