I know this subject has been covered, but what I didn't seem to find is an answer to this. When you experience temp changes does your grouping become erratic or does your grouping just climb up the target. From what I've read people say certain powders are temp sensitive but I did not see where they said "in what way" they are sensitive.
In my experience, it depends on the rifle. I've noticed that a hotter day generally makes the cartridges appear to have more powder. So you'd need to answer, what does your rifle do with a bit more powder? It might just move the POI, or it might really screw up your accuracy, or it might lock you gun up!
Here is how I deal with it, rightly or wrongly.
I always work up my loads with temperature in mind. For example, lets say I have a rifle that shoots consistently over a 1/2gr range of powder and I'm shooting in cold temps (colder than I expect to hunt in), I'd use the low end of the 1/2gr range, that way when I shoot in warmer temps, which will increase pressure a little, I'm still in/near that 1/2gr range. Likewise, if I'm working up a load in warmer temps than I expect, I'll pick toward the higher end of the range, because as temps drop, I'd expect it to drop down into the loads sweet spot.
The best option however, is to test the load in the conditions you will be using them.
AJ
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Groups open as it is akin to increasing or decreasing your powder charge, you just fall out of tune.
In some rifles the load window is a little broader than others, which may mask temp senstivity, and cause the group to elevate. The converse is also true.
I have a rifle that has a very narrow load window when shot in a lower velocity node. Just a few degrees in temp or a slight humidity change and groups go haywire. When the same gun is run at a higher node it is much more stable. The bad part of that scenerio is that the gun groups best at the lower accuracy node I have a 28gr or so load for that rifle that is amazing or terrible depending on conditions. I also have a 31gr load that just shoots pretty good, but is almost always basically in tune.
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When the temp gets above 90 my load of 80.0 of 7828 in my 300 WBY (210 Bergers) gets pretty up happy and starts kicking the brass pretty good, extractor marks get deep. Does not effect accuracy.
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I concur with what AJ and eddybo said. Basically, warmer temperatures will create higher pressures and higher velocities. Changes in velocity can cause changes in barrel harmonics, which can cause changes in POI. If you are still within the "node" or "sweet spot" for your particular rifle and load, you may not see any difference. If not, you could see large variations in POI or group size.
Recently, I tried to gather some data myself on temperature sensitivity. I developed loads for two different rifles that gave me a satisfactory node, using the OCW process. One was a .270, using IMR 4350, and the other was a .300 WM, using RL22. (not apples to apples, but I wanted to see the effect of temperature on my loads.) I measured velocities at ambient air temperatures from 40 to 80 degrees farenheit. The .270 with IMR 4350 increased approx 50 FPS from 40 to 80 degrees. The .300 with RL22 increased approx 70 FPS from 40 to 80 degrees. My POI's moved around a bit, even though I thought I had found a good node. At long range, there could be a significant difference. Bottom line, as already mentioned: test your loads and sight in under the conditions you expect to be hunting.
For me it seems that my rifle have grouped the same from temp to temp where as the long range POI has changed higher or lower.
My theory is that the powder charge weight creates as certain harmonic node in the barrel despite a ever so slight change in burn rate. This rate of burn change in very very small. I believe this beacuse certain powders have such a similar burn rate and almost always have the same velocity at the same charge weights. For example I have shot IMR 4064, VARGET and RL-15 at the same charges with VERY close velocities (within 20 FPS) and accuracy was similar. When going to a drasticaly different powder of the same charge weight, it is either conciderably faster or slower. A dramticaly different burn rate causes a different node hence the differing group sizes. This is just my theory. Every rifle and load should be tested in varying conditions so as to KNOW for sure how they will act.
PS, I have used one powder in my 300 RUM that increased velocity when the loads got colder.
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I have a 28gr or so load for that rifle that is amazing or terrible depending on conditions. I also have a 31gr load that just shoots pretty good, but is almost always basically in tune.
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