Being new to reloading, I hope this is not an ignorant question.

I am working on loads for a 7mm RUM that I will be using on an elk hunt in Montana during the last week of October. How much will the performance of these loads change over temp and altitude changes. I expect that altitude would be the least significant factor if even a factor at all, but how much will a given load change from 80 deg to 0 deg?

Also what about humidity?

Right now I am trying IMR 7828 and IMR 4831 with 175 gn. Noslers

Altitude will have no effect on the velocity of your loads, but it will have some small effect on the trajectory, due to the thinner air slowing the projectile less and drop therefore being a bit less. This is hardly enough to compensate for, but you'll want to check your zero when you arrive at your hunting destination anyway.

Colder temperatures will typically VERY SLIGHTLY reduce velocities. With the powders you mention, it might be on the order of 50 fps, but again, not enough to attempt to compensate for.

So, maybe the lower velocity due to the colder temperature will just about offset in the flatter trajectory due to the thinner atmosphere, and you'll be sitting pretty, anyway.

Don't worry, be happy. Just check your sights when you get to camp.

[ 08-26-2003, 00:59: Message edited by: Stonecreek ]

Here is a link to some good information:

http://www.riflebarrels.com/articles/bullets_ballastics/ballistic_altitude_temperature_humidity.htm

Byron, that Lilja article you posted is really a good one. Enjoyed the read. Thanks.

Barrel temperature and powder temperature each seem to have an effect on internal ballistics. Ambient temperature PROBABLY doesn't mean much, except for how it influences powder and barrel temperatures.

With that warning stated, figure about 1.7 fps per degree F ambient (yeah, I know... I just said ambient doesn't matter much... I assume no resonsibility whatever for this sorry state of affairs), with pressure and MV going up as temperature goes up. So if your temperature drops 30 F, figure you'll lose 50 fps or so. In many cases, that's equivalent to a grain of powder.

SAAMI pressures are specified at an ambient of 72 F.

You are probably worrying too much. I've not noticed a difference over any normal hunting ranges, although common sense tells you that altitude and high temperature will raise velocities. I have noticed that for some reason, don't know the science of it, but high, dry climates seem to elevate pressures somewhat, so be conservative when hunting in the western states and Africa. By the way, it may be damn near 70-80 degrees in October in Montana, you just never know. I hope not, but I've been in Wyoming and Idaho and hunted in shirtsleeves in September and early October, and the hunting is tough in this kind of weather. If it's like this be sure to bring a fishing rod so your trip isn't a total waste. Then again, it may be raining or ass deep in snow.

Thanks for all your help folks. I guess I an worrying too much. Just wanted to be sure I don't make a stupid mistake.

Be a bit more concerned with the loss of velocity due to temperature.

I've chronographed loads developed at 1300' elevation and 60� temperature at 2900fps. At 8000' and temperatures at 20� those loads chronographed @ 2720fps. There would be significant drop in your bullet at the longer ranges. Ballistics programs are available to enable to compensate for the differences in the less dense air as well as the drop in velocity dure to temperature. The Hodgdon line of Extreme powders are less sensitive to velocity drops than most of the other commonly used powders. Try to develop your load with them if your rifle "likes" them.

The Lilja article is excellent. ---- I develop loads at 365 ft above sealevel and hunt up to 10,000 ft and see no perceptable change in POI out to 300 yards. My loads at home are shot at 100 degrees at highest, my hunting has been done as cold as 20 below zero, once again with no perceptable difference in POI while shooting from .300 Win up to .358 STA. Good shooting.