How big a part does ambient temperature play in velocity and pressure? I live in SW Az, the mornings right now are abt 65 and the afternoons abt 105 degrees. It seems my loads will shoot much differently depending on the time of day/temp. If so, how much difference can I expect at these temp. ranges. It seems to change the trajectory significantly as well. What would happen if I worked up a load at 90-100 degrees and then in the fall, went hunting in 40-50 degree weather? What should I do?? Thanks, Mark H.

I think it's the temperature of the cartridge when it's fired. Boyles law states P=t/v where P is pressure, t temperature and v volume.

As far as what to do about hunting in colder temperatures it's nothing to be concerned about pressure wise of course but the point of impact at very long range could and should vary.

I have much the same situation, plus my range is about 1600' above sea level which also affects things. I find that with most propellants, including those touted to be not sensitive to temperature, I need to reduce loads in the medium capacity cases, (like 300 Win Mag, 338 Win Mag, 30-06, etc.,) one grain to equal the velocity generated under 70 degrees, when the temperature exceeds that. Consequently I just don't shoot when it's above that temp. Also, I've chronographed loads in Montana in freezing weather, that were 100 fps slower than my home range in warmer weather. Your concern with humidity is principally while you are loading as high humidity will affect the powder and weight as it is hygroscopic. In short, there is considerable effect and something which should be noted as loads are developed or used.

My experience is in line with Steve's data. On my first trip to Africa, I loaded up some 200 gr bullets for use in my 300WinMag. It was 2900fps in our summer temperatures-95-100F.

When we got to the mountains in the Cape the temperature was below freezing and we had six inches of snow by the time we saw any game. Unfortunately, we checked our zero when the temperature was more moderate.

Bottom line, I lost about 150-200fps when it got really cold, which is a significant change in trajectory.

I believe that temperature is the single greatest change one is liable to experience while hunting particularly those of us who develop loads in the heat and hunt in the cold. By the bye, if you work up loads in the cold and shoot in the heat, a sometimes thing in Africa, you don't want a max load unless you want to hammer a bolt open while dealing with a charging buffalo. Ku-dude