Powders and temperature change ?

[cjmac]

So for 223 and 308 iv narrowed my powder down to just , benchmark , imr4895 and win 748.. i know 4895 and 748 are more temperature sensitive . But how much csn i expect my group sizes to chance in different weather at times of the year ? With benchmark in the 223 it dont change all that much , just the POI . I only shoot o at 300 or less

 

[cjmac]

I have tons of powder im trying to narrow it down so i can buy in bulk... i have norma 200 ..TAC .. imr 4895 , 4064 , 8208 , RL15. Benchmark ... i guess its good to have different loads for when you cant find one powder

 

[SBS]

So for 223 and 308 iv narrowed my powder down to just , benchmark , imr4895 and win 748.. i know 4895 and 748 are more temperature sensitive . But how much csn i expect my group sizes to chance in different weather at times of the year ? With benchmark in the 223 it dont change all that much , just the POI . I only shoot o at 300 or less

I don't know if the question about group size can be answered except by trial & error. Some increases in pressure/velocity cause the group size to change more than others; often depending on the individual rifle. Logically, those with the greatest temperature sensitivity (therefore the greatest change in pressure/velocity), are the ones most likely to change group size. Years ago in LR HP competition, I could develop a load (using extruded IMR powders), check the speed through the chronograph and when the temperature changed substantially, I could adjust my load over the chrono to achieve the same speed and the group would remain much the same size.
In high volume PD hunts, I find it necessary to load in advance in much cooler conditions than I will be shooting the rounds. Those powders that are not temp. sensitive are a great help. However, I have quite a stash of older temp. sensitive stuff, including ball powders, to shoot up. I usually work out a load that is about a grain or so (depending on case capacity) less than max in usually 80's weather and it's never been too hot in 100 degree shooting and the groups don't seem to suffer enough to miss the dogs.
I wish you luck in trying to reduce your powder variety. I try, but seems like I have a larger number every year as new ones are introduced and it's hard to resist trying them out.

 

[M14AMU]

IMR4895 shot me many 'cleans' across a lot of years shooting the short lines in NRA HP Comp using the .308 Win! On more than one occasion the 10 shot group could be covered with a 2" spotter at 300 yards.

 

[searcher]

Whether your accuracy will change during temperature changes depends a lot on how wide your accuracy window is for the particular powder/load you are shooting. Since the width of the accuracy window will change depending on bullet, primer, etc., - even with the same powder, I found it worthwhile to chronograph my favorite loads when HOT and when COLD, record the velocities, then determine the amount of powder it takes to bridge the difference in those velocities. Then, if to be shooting cold, I know how much of an adjustment to make for that particular load - and so on. With W748 as an example, my experience has been about 2/10 to 3/10 grains difference (reduction) in powder required when going from 55 degree weather to 90 degree and perhaps double that for the .308 - in order to maintain the equivalent velocity. Guns and loads may alter this a bit. I had a load using IMR8208XBR that, with a particular load, had to back off one full grain grain in summer (from 40 degrees to 90). On that particular load, I was still in my accuracy window, which was really wide with that load, yet too much pressure for the gun.

 

[cjmac]

Whats the best way to test how much Accuracy window you have?