TAC is probably the most temp-stable of the Ramshot powders, with Big Game a pretty close second. Hunter and Magnum are also pretty good, but not like the two faster powders.
When I first tested TAC at zero degrees, the muzzle velocity of a .223 load averaged EXACTLY the same as it had at 70 degrees. Now that was definitely just pure chance, as loads almost never chronograph exactly the same even when chronographed at the same temperature.
One thing I always have to remind folks of when discussing temperature stability is that the characteristics of powders can change at pressures out of their normal range. One of the Ramshot lab-techs pointed this out to me during one of my visits.
His example was using Varget to try to get absolute max velocities out of 140-grain bullets in the .260 Remington, and called it "stressing" the powder. Varget is one of the most temp-stable of powders, but stress it beyond its comfort zone and it loses some of that stability.
