prm,
One of the problems with "temperature sensitivity" is that there really isn't a firm definition, and to a certain extent it's dependent on the cartridge and load. A chapter in the Norma manual mentions this, essentially stating that a very temp-resistant powder can be developed for any specific cartridge/bullet combination. But the same powder may not be particularly temp-resistant in another cartridge/bullet combination.
I've seen this many times during my load testing at different temperatures. Among the more cold-resistant results of what might be termed "standard" powders occurred with IMR4350. A load with IMR4350 and a 300-grain bullets in the .375 H&H lost only 38 fps from 70 to zero, but during the very same test, IMR4350 lost 74 fps in a .30-06 load with 165's.
During the same test, however, two loads with Hodgdon Extreme powders lost less than 15 fps in the 7x57 and .338 Winchester--which is less than the normal variation between two strings of chronographed shots recorded at the same temperature. This is what I've generally seen with Extremes in a wide range of cartridge/bullets combinations.
But the truth is that powders claimed to be temperature resistant just have a broader "comfort zone" than other powders. Another truth is that NO powder will produce the same velocity, even in loads in its comfort zone, at temperatures from zero to 100 degrees. And powders that resist cold may not do so well in heat, though in general there's an overall resistance to both cold and heat in the most effective powders.
The most temperature-sensitive powders I've tested, both in cold and heat, are those developed to produce higher-than-average velocities in specific applications, even inside their comfort zones.
Another factor is that large velocity variations often don't show up until temperatures get really cold or really hot. Almost any powder will be pretty consistent between about 35-40 and 80-85. Some that are more temp-resistant will extend that range a little, but it takes a really temp-resistant powder to minimize velocity variations in a wide range of cartridge/bullet combinations from zero to 100.
My own tests of RL-17 in various loads indicate its temp-resistance is better than average, but not outstanding.
John, I completely agree there is little definition. The powder is only one variable. Primers, barrels are also likely influential to some degree. I will admit I don't understand the physics/chemistry of how a powder could have little change across temps in one cartridge and significant change in another. A given powder is applied only in similar expansion ratio situations and would, I think, react similarly. I certainly can't refute Norma or anyone else though. Sure be fun to test though! I need more rifles. What I did is nothing more than collect a few data points. Certainly not enough to be definitive. My goal was to simply have a better understanding how velocity would change from an indoor range to a typical winter hunting situation. Nothing more. Having said that, it told me enough regarding how my loads perform in my rifles. It was very consistent in that ball powders lost a "significant" (to me) amount of velocity while the extruded powders lost very little if any. Off the top of my head, 2000MR, TAC, and LVR lost 80-100+ FPS, while 8208, RL-17, H4350, and IMR4451 lost anywhere from 0-25 FPS. Again, only four shots, take it for what it's worth.
