Pressure spikes and drops due to temperature have long been a bane of a reloader's existence. However, advancements in propellants have mitigated the variable of smokeless powder temperature sensitivity, in many respects.
To understand smokeless powder temperature sensitivity, a look back at cartridge history and development is in order.
In the early days of the 20th century, when cordite was the propellant du jour, the reputation of cartridges was made and/or broken based on their performance in the heat of the tropics.
You see, the firearms were regulated and pressure tested in the relatively cool climate of England and Europe, and were then carried off by brave sportsmen into the brutal heat of India and Africa. In that heat, the pressures spiked and extraction of cases became difficult, if not impossible. The cordite was the culprit, as the chemical compound was extremely sensitive to fluctuations on temperature.
The answer to the problem, at least temporarily, resulted in some of our most famous cartridge cases. The .416 Rigby, for example, was made with an oversized case to keep the pressures low when loaded with cordite. Those low pressures, and the ease and reliability of extraction, bestowed the solid reputation that the .416 Rigby had earned even before being launched into super stardom by Robert Ruark.
Modern smokeless powders solved much of that problem, giving not only a boost in attainable velocities, but a much more stable platform, however there is still a certain level of smokeless powder temperature sensitivity that rears its ugly head. The usual accepted value was a 1 fps gain or loss per degree Fahrenheit of deviation from 68-degrees. So, if you were to measure your velocity of say, 3,000 fps at 68-degrees, and were to retest, you could expect 3,012 fps at 80-degrees when chasing African plains game, and 2,950 fps at 18-degrees when hunting deer in Canada.
Now, to a hunter, if the accuracy was within reason, this could easily be overcome, but to a benchrest target shooter, this would be unacceptable, because it would open up the tiny groups they seek. Reloder-19, which I absolutely adore, has been among the chief culprits of this phenomenon. For this reason, I always developed my loads in the extreme heat of summer, so they would function anywhere in the world, from Africa to Alberta, with no pressure problems. Developing a load in the cold of winter could produce dangerous pressures in the heat of the Zambezi Valley.
The newer smokeless powder developments, like Hodgdon’s Extreme line, IMR’s Enduro powders and Alliant’s new Reloder 23 and 26 are all designed to have virtually no pressure fluctuation in a range of temperatures from 0-degrees to 125-degrees F, which pretty well sums up our hunting extremes, and certainly covers the benchrest crowd that are legally sane.
The Extreme line from Hodgdon has been with us for a while, and includes H-4831SC and Varget, both of which have shown to be fantastic powders, and perfect for magnum and standard cases, respectively. IMR 4166, 4451 and 7977 are a trio giving a burn rate that sort mimic (but are in no way interchangeable, don’t even think it!) with IMR 4064, 4350 and 7828, suitable for most, if not all, rifle cartridges. The Reloder 23 and 26 powders are along the lines (again, in no way interchangeable) of the proven Reloder 22 and 25.
The problem is that you’d have to work up new loads. Or, you can look at it like a new adventure, with more time at the bench, and that is never a bad thing!