Once again, the reason I prefer the term temperature resistant because there is no truly temperature-proof powder--yet.
The powders that have done best in my own tests have all essentially chronographed the same at zero and 70 Fahrenheit. Not many powders do that, but some do, notably all the Hodgdon Extremes I've tested, plus Ramshot TAC and Big Game.
But even all those powders tend to increase in velocity at temperatures over 70. The increase isn't as much as with standard powders, often only half as much. But apparently no powder company so far has figured out how to produce powders that are velocity (and hence pressure) stable in temmperatures over 70.
Some powders aren't quite as temp-stable as those I just listed bit better than average. In that class I'd put Reloder 15, plus Ramshot Hunter and Magnum.
The claims for temp-resistance aren't BS, as some have suggested, but there are limitations to the technology powder manufacturers now have. Plus, sometimes other variables will affect even the most temp-resistant powders. A ballistic lab tech (who has done a lot more temp-testing than I have) once explained that stressing a powder doesn't help its temp-resistance.
By this he meant using it for less than ideal purposes What he calls ideal is a load that fills the case and produces about all the velocity a certain case and bullet weight are capable of, at safe pressures. The example he gave of a stressed powder is Varget and a 140-grain bullet in the .260 Remington. While accuracy might be good, the powder charge won't fill the case or produce top velocities--and Varget won't be nearly as temp-resistant as it would be with a lighter bullet, or H4350 would be with a 140.
