Rifle chamber temperature has a much greater impact on pressure than ambient temperature. Denton Bramwell has done some well-instrumented testing of this. Search for his 2 papers on the topic; I don't have the URLs conveniently at hand or I'd link directly to them. I think 6mmbr has one of them on their site, and RSI the other, but I'm not positive. Sniper's Hide has, I think, at least one of them.

Quote Originally Posted by seanhagerty View Post
If you are using Hodgdons EXTREME line of powders, they are not sensitive to temp changes.
This is a commonly-held fallacy. In fact, temperature resistance IS NOT an across-the-board thing. The temp-resistance only holds within a design window that's to do with case capacity, bore diameter, bullet weight, etc. The harder you try to make perfect resistance, the worse the temp-resistance gets when you use the powder outside its design envelope.