Thank-you very much for that excellent data Tobartell - much appreciated.
Some decades ago, one of my kids did a science project on this - we put some cartridges in the freezer before testing, and some in a plastic bag, into boiling water before testing - and the results were much less accurate and valuable than yours, but did convince me that a gun needs to be sighted in at hunting temperatures or at least compensated for.
Recent shooting - in the past week - has shown reminded me that it really does matter - two rifles shooting three inches lower at -20C at 200 yards than they did when I sighted them in at summertime temps.
Most loads seem to behave best at maximum loadings, so the decrease in velocity at low temps likely does take them out of the sweet spot - but this would seldom affect hunting (I suppose.)
It would take some seriously careful data-collecting to record the actual case temperature and powder temperature and barrel temperature and bullet temperature - they likely vary a lot for various reasons. The first shot might be from a warm cartridge, and a very cold gun - but the second may be a colder cartridge from sitting open on the bench, and from a gun which is warmed by the first shot. Then, the last shot of each group is from the warmest chamber yet the coldest cartridge, so the temperature of the cartridge is dependent on the time that it sits in the warm chamber before firing. So.... cold temperatures really do widen the range of variables too much to account for except by the behavior of the particular type of powder at different temperatures, and sighting in the rifle at temperatures at which they are to be shot.
Coffee has kicked in, so I'll babble a while longer.
My general impressions have been that finer-grained powders which measure well and are more often considered target powders tend to be affected more by temperature variations ( I'll go back and consider Todbartells data now) than the old log-grained powders - and shotgun/handgun powders are affected most of all - I've had as much as 10 percent velocity loss at - 25F in 45acp. This is enough to prevent the second ring of the cowbell if the recoil spring is fresh in a handgun - and may have to do with bullet lube type, and the the function of a semi-auto handgun or shotgun may depend on it being dry of oils in the action. (A tiny bit of lube on locking lugs being okay, the rest of the gun just gets WD or the like maybe.)
One last thing - when the primer goes off, how much it immediately warms the powder might make the biggest difference of all - so we might try testing the different primers - standard and magnum - at temperatures - this may make more difference than anything else, and the speed of the "warming" of the powder by the primer might be affected by the shape of the powder and how compressed it is in the cartridge.
This is our life - a search for the security of rules, but we find only more variables which make our rules invalid and us less certain - we become a listening child - knowing that we know little, but a seeker of some great Father-truth. A Nativity perhaps.
Merry Christmas to all.
