This is a topic to which I have referred in the past, with some helpful contributions from the technicians on the BBS.
I noticed, earlier this summer when the weather was especially warm, that my Webley Omega was especially impacted by the temperature, which was then over 30C. I first noticed this last year. I have fitted aperture sights to the Omega and I find it an accurate air rifle and am very happy with its performance.
However, in hot conditions, it is necessary to raise the sights because the power drops, to the extent of, say, nearly 2 inches at a range of 25-30 yds. Yesterday, when I used the rifle again in much cooler weather, it had restored itself to its usual format. It appears, although I cannot be sure, that there is a tipping point at which the velocity drops markedly. Extensive testing would have to be conducted to find out at what point this occurs, both to quantify and qualify the variations in performance. In this case, I have simply decided not to use the rifle in hot weather, which deals neatly with the problem.
I first noticed the difficulty with my BSA Airsporter in the late 1970s. I attributed it, then, to a possible expansion of the metal within the barrel. This was probably the least likely cause of the power drop but, again there was a tipping point. More likely, as others have very occasionally reported on this site, there are two other more likely explanations:
a) The air is less dense (ie it becomes lighter) in hot weather, which leads to a reduced 'punch' in the cylinder as the piston and washer drive home.
b) The cylinder (or washer) itself expands slightly. This might lead to an expansion within the chamber, causing the washer to pull against the chamber walls. Alternatively, the expansion might be in the other direction - ie outwards - in which case the washer would form a looser fit and experience possible seepage.
It would further appear that different air rifles display different effects. Some rifles, it would seem, are relatively unaffected. Others, much more so.
It is peculiar that no one appears to have conducted any testing to see what happens and how the influence of temperature can be addressed. One would think that the big airgun manufacturers would have conducted extensive testing. Perhaps they have but have not reported their results.
The impact of hot weather has an influence on power and, therefore, on accuracy, so it cannot be underestimated.
I should be interested to know what others think. Perhaps this should be posted on the General Forum of this site, to attract greater input. It appears that PCPs are unaffected by the phenomenon.
It might be that the problem is too complex to be answered by the industry and has therefore been ignored. On the other hand, it would be helpful to see any research that exists, either here, on the Continent or in the USA.
Rgds
A