Thanks.
My experience is that the temperature of the rifle does affect velocity. I have tested ammo both ways, by heating.cooling the ammo only, and by firing in outdoor conditions from below zero to 100F, with the ammo often heated a little more by "greenhousing" it in a clear plastic bag. Sometimes frozen ammo works about like it does in actual cold, but more often it doesn't.
Might also comment that I've tested many other powders that you didn't list. Even some supposedly temperature sensitive powders will often perform very consistently in cold in specific applications, and sometimes some supposedly temperature-insensitive powders won't. Have discussed this with some experienced lab ballisticians, and they have experienced the same things. The guy at Hodgdon says the Extremes work best with a full case, and the right primer for the case, which doesn't necessarily have to be a magnum primer. But have also gotten very good cold results from Ia compressed load of MR4350 in the .375 H&H and 300-grain bullets, as good as any with Extreme powders.
Have also gotten result similar to the Extremes with the IMR Enduron line from 4166 to 8133. One other powder that so far has performed very coinsistently in subzero cold in cartridges from the .243 Winchester to 9.3x62 is Ramshot Big Game, in cartridges under about 55 grains in capacity with standard primers, and with magnum primers in larger ones. Ramshot TAC also works very well in specific rounds, such as the .223 Remington and .308 Winchester, which isn't surprising since it was originally designed as a military powder for the 5.56x45 and 7.62x51. Taken outside that "comfort zone," however, and it sometimes doesn't perform as consistently in cold.
