Object Lesson on the effect of temperature on POI

[Dave R]

I hit the range yeaterday to try some new .308 loads. Learned something about the effects of temperature.

I had my scope adjusted for my favorite Milsurp (FNM) at 200 yards, so I started with a group of that at 200 yards to check things out. Group was under 2", centered an inch low and an inch right from POA (there was a light crosswind). The ammo had gone from my house to the heated car to the range, so it was near room temperature for that first group. Range temp was about freezing.

Moved on to shooting handloads, trying different power charges. Now, it has been so long since I've been to the range that I last shot similar loads in August. That's when I loaded these.

After about 45 min with handloads, I shot another group with the Milsurp. POI was 4" below the first group. I presume that's because the powder was down around freezing. Call it a 35-40 degree (F) drop in temperature. I was using H335.

Also, all the othe loads I loaded in August shot lower than I remembered. And the load that was most accurate in August--wasn't in February. Instead, a load slightly lighter was more accurate. Actually, that may not be significant because I was playing with crimps, so the difference may be due to crimp more than temp.

So I have question. We develop a load for accuracy based on barrel harmonics. But in winter (or summer), velocities will change, and the the "density" of the barrel will change with temp. So does that mean the most accurate warm-weather load may not be the most accurate cold-weather load? Do I need summer and winter loads? Or have you found that the accurate loads are accurate regardless of temp?

 

[30Cal]

You'll notice, if you keep a reasonable amount of data, that some powders will be more insensitive to temperature than others. Varget is supposed to be pretty good in that respect. I haven't noticed any decent loads that go sour beyond usefullness depending on the temperature, but that's not to say it can't happen.

When I shoot slowfire in a match, I press the round into the magazine and only chamber right before I'm about to squeeze the trigger. It's my way of making the conditions of the ammunition as consistant as possible rather than leaving it in a hot chamber for a period of time that will always be different from one round to the next.

I know a couple match shooters that weigh their assembled cartridges and shoot the heaviest ones early in the day (when it's coolest) and the lightest ones latest in the day (when it's hot). The heaviest loads will have the highest pressure and the lightest loads will have the lowest, be it due to case weight or bullet weight.

 

[Paul "Fitz" Jones]

Reading Primers

I have swapped the navy ship shooting teams Star Reloaders and California Saeco bullet casting tools back in the 70's because their small arms ammo was taken by the army for use in Nam and abandoned there so they contacted me and I got about a hundred thousand once fired GI 45acp brass that never hit dirt. Well my scouts sorted it out by maker and year into my Fitz ammo boxes and I sold it to top competitive shooters from the 1970's to date.

My point is in seeing and sorting many thousands of brass all identical WCC71 through WCC79 that were supposed to be identical the primers showed differing expansion and indentations and I think the only variable was when the ammo was fired on a cool day as opposed to a hot day. So the primers reading more pressure were either stored in the sun before they were fired or it was a hot day when they were fired so temperature had to have something to do in the differing pressure reading of the fired primers. My main sources for the brass was San Diego and Guam ships.

Paul Jones