A couple comments:

Shrapnel almost always jumps into these threads, saying he's used H335 for many years in his .223s and never had any problems.

But one thing he doesn't mention is that he HATES hot weather. I have shot PDs and "gophers" (Richardson's ground squirrels) with him many times over the years, and cannot remember ever doing so on a day over about 80 degrees.

Might also mention that I have tested a bunch of powders at temperatures from zero F. to around 100 degrees--and by that I mean in actual temperatures including the rifle, not just chilling or heating the ammo, which only rarely produces similar results. The velocities of the best temp-resistant powders don't change much at all from 70 down to zero, and even somewhat below zero. But they ALL result in faster velocities (and hence higher pressures) at temperatures above 70, usually becoming more obvious around 80.

The more temp-resistant powders, however, do tend to gain less velocity/pressure at over 80 than "standard" powders. Also, even the most temperature-sensitive powders (among which I'd list H335) don't vary much in velocity from temperatures in the 20s to around 80. Or at least none of the presently produced rifle powders I've tested have.

All of this is why I prefer the term temperature-resistant, rather than temperature-insensitive. None of 'em are totally immune to temperature, especially heat.

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