Well it seems that your load as you say is not that over compressed. I'm getting between 99-102% of case capacity for 264 Win Mag at 80 gr LRT depending on what brass you are using. But, it must be that this is such a slow burner at .285 burn rate for LRT thatIn my humble opinion, unburned powder may have come from a way over compressed load, or maybe not using a magnum primer, or both with a powder that is very slow burning to begin with......this was not because of a temperature difference between 101 and 87 degrees.
Maybe if you said 30 below, I'd more believe it. But, free powder may have been going out the barrel at 101 too, just the wind wasnt blowing it back into you that time.
I will do a quick calc in QL to see what percent of over fill that load was in 264 Mag. case out of curiousity.
in a compressed load, if you are not using a mag primer, and maybe if you are not seating your bullet very deep that it will not burn all the powder. I would seat the bullet at a depth that is still safe but gives it a little more burn time and use a mag primer, and then if you still get blow back of unburned powder, I guess that's just the way it is. But I don't believe its temperature related.
There are only a few powders that burn slower than LRT. US 869 0.2735, RL-33 at .267 and N570 at 0.277
I've heard people using 869 say its like shooting tar its so slow. So, if you have compressed loads, regular primers, long coal's with
bullets seated fairly shallow, and a really slow burning powder, I suppose you are gonna get some blowback of unburned powder
possibly. Either that or its minute traces of carbon from your rifle barrel? Do you clean it often?
Just some thoughts of things to consider as you ponder this.