There are a lot of reloading powders out there, and they all work to make ammo that safely goes bang as long as you do your part. For years, the subject of “temperature sensitivity” was more about knowing how your handloads would perform in different climatic conditions rather than picking some high tech super powder to allow you to ignore climatic conditions.
Not anymore. Powder manufacturers and sellers (not always the same!) have taken to advertising some powders as “temperature stable” or “temperature insensitive.” And like all good advertising campaigns, there is an element of truth. But it is mostly hype, and here is why.
Even the most “temperature stable” powder is still going to show temperature variations in velocity and pressure. Obviously more “temp stable” shows less variation, and less “temp stable” show more variation, but the point is that (for example only) you’ll get the same pressure and velocity shift with SuperPowderXtreme with a 40 degree shift that you’ll get with OldStandardPowderZ with a 20 degree shift. Clearly the SuperPowderXtreme is “twice as stable” than the OldStandardPowderZ, but what does that REALLY mean?
Secondly, temperature stability depends on the expansion volume of the cartridge plus rifle bore. Powders that are extremely “temperature sensitive” in 243 Win are going to be much less so in 358 Win because of the much greater bore volume. If you are hunting with a 444 Marlin or 45-70 I wouldn’t worry about temp sensitivity as long as the loads are under max.
So what is the benefit for hunters? It really means that you can handload ammunition on the slightly hotter side of the load chart and not worry about extraction issues in hot weather. That’s it. At hunting ranges for big game, it doesn’t mean a thing. Even a 1 fps/degree “temp sensitive” powder is only going to have a 40 fps difference with a 40 degree swing. As long as your hunting load isn’t the top max charge developed in the dead of winter and then taken to a 120 degree jungle (F, not C for my international readers) even OldStandardPowderZ is going to be just fine.
The old “do your load workup in Summer, then confirm zero before hunting season” is still darn good advice, even with SuperPowderXtreme temperature stable powders.
The other big application, target shooting, does see a little more benefit to more stable powders. The old High Power service rifle teams used to prefer IMR4064 to IMR4895 because it was more temperature stable across the competition season and heat of the day. This means fewer “temperature correction clicks” on the sights, which means less work load on the shooter. There is benefit to this, although the military loaded up millions of rounds of M72, M118SB, and M852 match ammunition with IMR4895 and did just fine with it.
Another target shooting benefit for temp stable powders is long range shooters. The barrel and chamber temperatures have a greater impact on pressure and velocity than atmospheric temperature. And a 40 fps difference may only be fractions of an inch at 200 yards, becomes several inches to feet at 1,000 yards. It’s a real benefit, and helps keep groups tighter throughout a shot string. Although even “temp stable” powders will need a few clicks to re-zero the rifle with a swing between 40 and 90 degrees simply because long distance really magnifies even small muzzle velocity variations.
So what do you do when Varget doesn’t group as tight as IMR4895 or Reloader15 out of YOUR particular rifle? That’s up to you to decide. You could try IMR4166 or H4895, or some other “temp stable” powder to see if you can get the accuracy you want with a more temp stable powder. Or you can use IMR4895 or Reloader15 and keep an accurate data book on how your rifle and load perform for you in different conditions. Neither answer is wrong.
I hope I’ve made the case that “temperature stability” isn’t the end all of choosing a reloading powder, it is true that it has benefits but those benefits are mostly irrelevant to hunters, and only offer a small advantage to target shooters (many a Palma competitor has won with IMR4895 and 155gr BTHP bullets). It doesn’t hurt to have it of course, but it also won’t win a match for you.