This is an old thread, and I am resurrecting it as some things were not discussed. I shoot in extreme temperature variations, both environmental and the temperature of the rifle, and thus the ammo. Central Montana is one of the worst places on Earth to shoot at long range if accuracy is the issue.....and long range shooting is the norm here. You CAN jump a whitetail in willow thickets by streams in nearby mountains....but that is NOT where the big mule deer are found.
Temperature sensitivity at ANY environmental temperature can be checked by firing a long string of shots at 1 minute intervals and checking the velocity with a chronograph. Immediately after firing a shot, place the next round in the chamber to let it "cook". The absolute worst temperature stability I have experienced is with IMR 3031 where with a 10-string shot I started with moderate .223 Rem velocity and ended the string with high end 5.56mm NATO velocities. FPE was well into the 1300's of foot pounds at the end. Every single shot had a higher velocity than the previous shot. This is practicaly unheard of with a temp stable powder.
Ball powder is not always as temp sensitive as some may claim. The military ball rounds I stockpiled were both made by FNM in 5.56mm and 7.62mm NATO, in the standard 62 grain and 147 grain bullet weights. Both use small ball powders and both show almost no velocity variation when doing long strings of shots. Velocity variations from shot to shot are due to the unequal powder charges, which are always thrown instead of individually weighed in military standard issue ammo. Bullet weight varies a couple of grains also. By about 4-5 grains if it is .308 cal 147 grain and supplied by Winchester. However, average velocity stays fairly constant with temperature variation in FNM military ammo. These are better performers than their domestic LC and WCC competition.
Hodgdon Extreme powders do rather well here, and I am converting many loads over to those powders. Ramshot is another powder line that has gotten my attention. Being Belgian, I would expect some of the Ramshot powders to be what was used in my FNM military surplus ammo. Ramshot TAC is especially noteworthy in .223 Rem applications. Especially if you want to throw charges without weighing them for volume shooting aplications such as prairie dogs.
H4895 and Varget often beat all other contenders in .223 Rem and .308 Win for me, with one offering slightly better accuracy while the other offers slightly better velocity, depending on bullet weight. I have yet to see any pronounced temperature sensitivity with either powder when firing long strings of shots. The downside is that charges must be individually weighed as neither one meters well. I generally choose a powder that gives a slightly compressed charge, as this provides consistency in "lightup" with no chance of "flashover" as well as giving no variation if the shot is angled up, down or level. I never shoot loads that fill the case below 90% capacity. 80% capacity can have a higher peak pressure than a compressed load, even if the velocity is low. Compressed stick powders generally give the lowest extreme spreads in velocity. I like to hear the "crunch of quality"when seating a bullet.
I have worked up too many loads at one temperature to have it shoot much worse at a widely different temperature. The Ultimate Sniper Manual says to change your zero with every 20 degree Fahrenheit change in temp and every 1000 foot change in elevation. You can't do anything about the change in air density with altitude variation. However, with the right powder AND load density, the 20 degree Fahrenheit rule does not necessarily apply. Notice I said LOAD DENSITY. A powder that in temp stable will STILL show velocity variation if the case is underfilled and you shoot up, down, or level. It has a different "lightup" in each situation, and that has NOTHING to do with temperature stability. Get ALL your ducks in a row before looking at temp stability only. Slight compression of powder, especially stick powder, seems to add stability to a load. A Lee factory crimp has also become standard, as bullets that are merely pushed into place will have different release forces depending how long the rounds have sat around. When pulling bullets that have been loaded for a long time, I can feel the "pop" as the metals shear apart. A round that was loaded yesterday will have a much lower release force and no "pop". A Lee factory crimp adds a more consistent release force as the presence or absence of metal bonding over time is swamped out by the factory crimp.
