The "new generation" R15 developed for the military contract is as good as anything made in the heat. SAAMI specs call for testing up to 140 degrees, and R15 is well within the specs on extra pressure there.
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<br>My tests in cold showed significant variation in some powders, perhaps because mine were done outside, at zero, with the whole rifle and cartridge kept overnight beforehand to replicate field conditions. Most other tests I've seen were done in labs, with the ammo chilled in dry ice or a freezer. It always seemed to me that "real" conditions might have greater affect, and they did.
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<br>The stars in my cold tests were the Extremes, the Ramshot rifle powders, and new R15. All others showed various amounts of velocity loss, some pretty steep. I've performed this experiment a couple times and lost up to 150 fps with some powders, with significant changes in point of impact, say 3-4" at 100 yards. This might not show up in a lab, shooting a warm rifle with cold ammo.
