As for Temperature stability, I’m on the Central Gulf Coast of Texas, about 120 Miles down from down town Houston and about 82 miles up from down town Corpus Christi. It seldom gets much colder than 30 degrees here, although the coldest I’ve seen it was Christmas Day 1989 when it got down to 8 degrees. I’ve seen it from12 to 20 a few times but that’s usually the exception and not the rule. However, I probably won’t be out shooting in anything colder than 30 if that. But extreme heat is a different story. The hottest I have ever seen it in Port Lavaca Texas was 110 degrees F on September 05, 2000, but that too was a fluke. In Summer it gets up to 95 and maybe as high as 100 on average, so high temperatures can be an issue.
Back about 40 years ago I was working up a load for my .270 Winchester with a 130 grain bullet and IMR 4831. I worked this load up in either February or March of 1980 when the ambient temp was around 50-70. Being new to reloading at that time, I thought nothing of ambient temperature. I ended up with 57 grains IMR 4831 under the 130 grain Sierra Game King. I took it to the range and shot several groups with it at that time with no signs of excessive pressure. But then a couple months later I took it to the range to see if I could improve my groups. That was in May with temperatures up in the low 90s. When I first shot it, I noticed a smoke trail to the target. Then when I tried to open the bolt it was pretty sticky and I had to bump it open with the heel of my hand. Later I noticed the primers were flattened pretty good. Like I said, I was new to reloading and at first I didn’t have a clue until I thought about what I had read in my Speer Number 9 Reloading manual about flattened primers and sticky bolts. (However, neither before nor since have I read anything about or experienced anything like the white vapor trail to target.) The first thing I did was cut the load back from 57 grains about three grains to 54. Then I loaded ten rounds and went back to the range. No more sticky bolt or flattened primers. However, it was about 250 FPS slower. So going by guidance from the Speer book I began working the load up again slowly. I got up to 56.9 grains before I noticed the bolt getting a little sticky again and the primers trying to flatten. So I dropped it back down to 56.7. At that level I saw no pressure signs and checking the diameter of the case head with a micrometer showed no expansion. So I’ve called that good for the past 40 years and had no problems. Had I been a more experienced reloader at that time I would have dropped it back further to about 56.2 or 56.4. But the proof’s in the pudding I guess and after 40 years with no problems I’m calling that good. I could probably get by with another grain of powder in the winter, but this load gives me 3100 FPS so why push it and take a chance on shooting one of those hotter loads in summer and experiencing another high pressure event? I'll just call what I have good.
