There have been a number of references to hunting loads developing overpressure problems in hot weather. It may be worth a discussion.

Firstly, it is more likely to get into high pressure problems from a round sitting in a hot chamber than from hot weather. If you fire a few rounds, chamber another one, and let it sit in the chamber for a while, it can get very hot.

Accordingly, I test all loads by letting them cook in a hot car for a while and testing. Then I intentionally heat up the rifle by firing and let the next test rounds cook in the hot chamber for a while before shooting. If there are still no indications of overpressure, I am comfortable with the safety of the load under field conditions.

In most high intensity hunting rifles, I try to get as much velocity as possible consistent with safety and accuracy. Testing for accuracy is deceptive, though. If you load XX grains and get a 1.0" group, and then load XX+1 grains and get a 1.5" group, you have not shot nearly enough groups to statistically validate an accuracy conclusion. Odds are, there is no real difference in accuracy between the two loads, and even if there were, it would not be significant in the field. In testing loads for some dozens of hunting rifles, I have seen few where small changes in powder charges made a statistically significant difference in accuracy. There have been lots of apparent differences, but it is kidding oneself to conclude that the differences are real, particularly when you consider variables in temperature, wind, bench position, and hold, not to mention trigger control.

I use good hunting bullets and drive them as fast as I can consistent with the above heat test and other safety indicators, and reasonable field accuracy -- consistently 1.5" or so.

If I get 1/2" groups with xx grains then 1-1/2" groups with xx+1 grains I shoot several more groups of each load.If the load is that sensitive I will not use that load at all.My own hunting loads show no significant change in group size when the load is increased or decreased by a grain or two.When developing a load for a 300 ultramag I ran into one load that more than doubled by increasing or decreasing the charge by one grain.I shot several groups with the same results.When the temperatures cooled later in the year I tried the load again and the group size increased to almost double compared to shooting in warm weather.Needless to say I will not use that load.

Howdy,

Occassionally, I have to test in HOT weatehr 100+ degrees. I place all ammo in a small cooler with an ice pack for the trip to the range. I take out only what I will shoot before the next barrel cleaning. The rifles stay in the air-conditioned vehicle until time to be shot. My wife says I treat my rifles better than I treat her! I just think you can't be too careful in this HOT Southwestern sun.

Coach

I've used H870, N560, Reloader series powders all of which are temperature sensitive; some more than others. Hot loads became excessive between winter and summer. Some experienced reloaders have advocated seasonal loads. That is one for summer and one for winter.
With the newer Hodgdon extreme powders this difference really is negligible. The consistency is much better and they are the only powders that I now use. With the exception of Retumbo you give up some velocity (50fps) because they are not double based. Temperature sensitivity is now a mute point.