RonAKA said:
So how do you compensate for this, and predict it ahead of time?
Very simple: you test it!
What people notice based on air temperature isn't the best indicator of temperature sensitivity. Rounds brought from home in a plastic case, transported in a heated/cooled vehicle will often experience very little change in temperature internally. Unless the rounds are soaked in an exposed manner, at temperature, for at least two hours, you don't really know the powder is at that temperate. The first time I ran this type of test, I put a thermocouple inside a loaded round (without primer) through the primer hole to see how long it took for the internal temperature to be the same as external.
The way the tests works is that you soak 3 batches of ammo at hot, cold and ambient temperature for at least two hours (5 rounds of each). I use a cooler with a heating pad for hot, the freezer in the clubhouse for cold (a cooler with ice works too), and just sit the rounds out in the open for ambient. Temperature is measured with thermocouples and/or mercury thermometers.
You then fire them one round at a time in a round robin sequence (ambient, hot, cold, ambient, hot, cold...). You let the barrel cool in between each shot. When you fire each round, you just transfer that round from the hot/room/cold storage to the chamber and fire it through the chrono as quickly as possible. You don't want to give the rounds any time to change temperature internally due to contact with the chamber. I set the scope on the lowest magnification and just make sure I'm sending the round through the sweet spot of the chrono (no groups here). You basically want to be the Jerry Miculek of loading a boltgun and firing it through a chrono. This is best done on a short pistol range, close to the berm.
If you're interested in the powder's temperature sensitivity, you don't soak the entire rifle, that isn't scientifically correct because you're mixing multiple effects. Thermal expansion/contraction of the barrel due to temperature is a different effect than powder temperature sensitivity. In this test, we are only interested in the powder's effect (primer too, as it cannot be isolated). By using the round robin sequence and letting the barrel cool in between shots, you're removing its temperature effect as a variable in the test. If you then wanted to take it to the level of testing it as a system, you would know how much the powder is contributing. Otherwise, you cannot isolate it from the barrel's effect. Then, you don't know how much each is contributing to the problem.
Here are some 308 Win. results:
It isn't a difficult test to run. However, if you don't have the ability to measure pressure (I do), I would advise putting the heating pad on a lower setting.
