Temperature and gunpowder!

[denton]

When it is cold, your rifle and your ammunition are big "heat sinks" that rob energy from the propellant gasses. It turns out that the temperature of the rifle is about 3X as important as the temperature of the ammunition. As your rifle warms from shooting, you'll see pressure and muzzle velocity rise.

As nearly as I can determine, the mechanism behind temperature compensating powders is that they burn faster when pressure is lower, so the quickness of the powder changes with temperature, offsetting the loss of energy.

The good news is that when temperature compensating powders work, they can work pretty well. For me, Varget gave practically perfect temperature compensation in my former .308, with a 46 grain load.

The bad news is that a powder that is well compensated in one cartridge may be poorly compensated in another. Most likely, this is because each caliber has its own amount of case volume, casing and bullet mass, and relative surface area exposed to the barrel steel. In the 223, Varget gives me terrible (lack of) temperature compensation. It is more temperature sensitive that commercial "military" rounds sold over the counter.

So, net of nets, when it works, it works. Quite often, it doesn't.