[jmr40]

It depends on which cartridge. Generally Varget is MUCH less sensitive to heat in rounds like 308. But many powders that are less sensitive to heat in one cartridge may not do so in others.

In 308 and many other cartridges expect Varget to change no more than about 1/2 fps for each degree of temperature change. It works both ways. Develop a load at 70 degrees and it will be slower at -20 and faster, and possibly over pressure, at 110. RL-15 is very similar to Varget BTW.

Powders like 4064 will change velocity 1.5-3 fps for each degree of temperature change.

I use all 3 powders in my 308 loads. To be honest with the best loads I can't tell a bit of difference in velocity and very little in accuracy. With the right loads I believe 4064 might be ever so slightly more accurate and at temps between +20 to about +80 there isn't enough velocity change to worry about. I tend to use 4064 when I'm trying to squeeze out the last bit of accuracy, but use the others in hunting loads where I may encounter more temperature extremes.

If I were getting significantly better accuracy and/or speed with 4064 I'd use it anyway and just deal with the loss of velocity in cold weather. As long as you're not right at the upper limits of, or over, book loads going over pressure in hot temps won't be an issue.