Hmm. Due to quite a bit of experience with Chronys, I suspect part the "problem" might be with the chronograph.
Its always easiest to blame the Chrony. I know you have done the same test as me, a chrony in front and behind a Oehler 35P and found not much of a difference.
Has anyone else experienced this? I ran nine rounds through my chronograph today without stopping pretty much to just use up these last nine rounds,,, but I wanted to get a velocity ,,,,the first three shots averaged about 2660 OK that's good I kept firing and each shot after that, the velocity slowly went down. Down to 2505 now this is a very pencil thin lightweight barrel and the barrel got pretty hot so my question is as the barrel gets hot should the velocity go down like that? I've never seen this before, but I never chronograph nine in a row in rapid order .
So he has a pencil barrel that he got hot. Velocity goes down so blame the chrony when there can actually be more to it. Thermal expansion is a real occurrence in rifle barrels and directly to rate of fire. Depending on the thickness i.e pencil to heavy varmint the effects show up or are negligible.
Anyway heres the formula, and test rifle was 22LR.
The question whether temperature changes have a significant impact on the hitting point should be answered. From a physical point of view, the rifle barrel is a heavy walled tube (in the case of the tested rifle: inner diameter 5.6 mm and outer diameter 22 mm). The changes in the barrel temperature cause the barrel to change its dimensions in the process of thermal expansion of solids. How important is this change? In the case considered here, the dimensions of internal and external diameter of the barrel change according to the formula for linear expansion of solids, i.e.
Φ = Φ0(1 + α ⋅ ΔT)
where Φ0 and Φ are the dimensions of the considered barrel diameter before and after shooting, respectively, ΔT is the increase in barrel temperature and α is the thermal expansion coefficient of steel equal to 1.2·10−5 1/.
The inner diameter of the barrel, with an increase in temperature of 5°C, would change from nominal 5.6 mm to 5.60034 mm, i.e. 0.006%.
Getting people to think outside the box for explanations can be tough.