Reasonable shift in POI in low temp

[axxxel]

I just came back from the range with my rifle, a 308 winchester. My impact (average over several three shot groups) was unexpectedly about 1MOA low at 300 yards.

I usually shoot this exact rifle at this very range, but it's been a while since I took it out. The rifle is zeroed at 60-70 F with 155 nosler bthp, shooting prone off of bipods.


It's freezing around here - about 5-10 degrees.

Is it reasonable to get a 1 moa shift in poi at 300 yards from a 50 degree difference in temperature?

 

[eastbank]

powders can be affected by temperature and can cause a change in fps, a 1 moa shift at 300 yards would not bother me.

 

[someguy2800]

I do a lot of chronograph testing down to like -20 F. Not unusual to loose 300 fps muzzle velocity going from 70 deg to sub zero. Some powders and primer combinations are much worse than others.

 

[someguy2800]

One thing I’ve learned through testing is that just because a certain powder is not temp sensitive going from say 60 deg to 100 deg, does not mean it will be insensitive going from 60 to 0 degrees. For example one of the most cold sensitive powders I’ve tested was H4831sc, which is often claimed to be one of the least temp sensitive powders on the market. Primers make a big difference in the cold.

 

[Chuck R.]

As somguy2800 said, the only way to know is to take multiple chronograph readings with varying temps, especially for your "long" shooters.

Ballistic program like Strelok will use the current weather data and adjust for your expected MV delta along with the conditions. Mine when linked with my Kestrel is pretty accurate, as long as I have good data going in.

Normal range hunting rifles, the 300 and under rigs, I honestly don't sweat it. I have switched all of my longer range rifles over to the Hodgdon "Extreme" line of powders to mitigate the effects as best I can and I've added a digital thermometer to my chronograph case. This time of year is great for getting readings during the "extremes".

Once you've got enough readings you can see the effects using a ballistic program, or just enter a couple different MVs for your favorite rifle and watch the delta in your drop chart.

I learned this years ago when a favorite load I developed while in Germany in the fall, resulted in s sticky bolt and flattened primers in the KS summer. It also went from 3100 to over 3200 with a 130 grain over my Oehler 35P.

Another way to mitigate, is to zero as best you can under similar conditions. There's some other variable such as pressure, humidity, and wind to account for.

Here's a decent article that describes some of the effects with some testing:

https://www.handloadermagazine.com/rifle-powder-temperature-sensitivity

Note, the author ^ also observed occasional horizontal deflection due to temp effect on MV.

And another:

Powder Temp Stability: Hodgdon Extreme vs. IMR Enduron

The goal of this field test was to quantify the temperature stability of the popular Hodgdon H4350 a...

precisionrifleblog.com

Bryan specifically cited Hodgdon Extreme Series powders as one of the least temperature sensitive powders he’d used. He also made it clear that no powder is completely insensitive to temperature. Your muzzle velocity WILL vary based on ambient temperature, regardless of what powder you use. So while we can seek to mitigate that variance … at this point, no powder allows you to completely eliminate it. But, you can minimize the effect by making a wise powder selection.

 

[sage5907]

The only so called temperature stable powders is I have tried are H4350 and Reloader 16. After making several trips to the range I stopped using these powders because I concluded that I was getting smaller groups with IMR 4350 and Reloader 17. I have always tweaked my final sight adjustment when the temperature is about 50 degrees and that allows me to hunt at any temperature from 20 to 70 without being concerned. Here in Oklahoma early spring is a good time to zero a rifle and I didn't say the wind wouldn't be blowing.

 

[jmr40]

Somewhere around 1-3 fps velocity change for each one degree temperature change is normal for MOST powder. That would mean 50-150 fps slower with a 50 degree drop in temperature. 1 MOA at 300 yards is 3". That would be pretty extreme. I'd just have to run the numbers through a ballistics program to see if 150 fps less at the muzzle would mean 3" more drop at 300 yards. I suppose it is possible.

Some powders are more stable in a wide range of temperatures. If you choose those powders around 1/2 fps for every 1 degree temperature change is the norm. Varget and RE15 are known as good powders for 308 that are stable over a wide range of temperatures. With either of those I'd only expect around 25-30 fps velocity loss.

The only so called temperature stable powders is I have tried are H4350 and Reloader 16.

The cartridge you choose also determines how sensitive powder is. H4350 is very stable in 30-06 whereas IMR4350 is not. Neither would be stable in a 308.


For most people it isn't something to worry a lot about. I find IMR4064 to be every bit equal to Varget as to accuracy and velocity in my 308's. But Varget is more stable. I PREFER Varget by a slight margin only because of that. If there were a significant difference in accuracy or velocity in favor of 4064 I'd use it and deal with the velocity loss.

 

[tmd16556]

It’s not just powder burn rate but also air density. Cold dry air will provide more resistance than humid warm air. I don’t know which is the dominant factor. Both impact things in the same direction and I suppose powder burn rate matters most at short ranges and drag changes start to matter at distance.

 

[JFrank]

3 inches low at 300 is a lot of variance for a 308 that’s in tune, mine don’t do that..

Out of tune rifle …. Well who knows.
Post some load data and you may get an answer.

 

[axxxel]

45 gr of vihtavuori N140 behind nosler cc 155 gr.

I'm thinking this poi shift is a bit much, although it is in the realm of possibility that it's ,"just" the air density and powder temperature.

The mirage from my suppressor was very noticable after just a single shot. I was shooting at a bunch of 8"x8" square targets with 2" black centers (airgun targets) and after just one shot I had to aim at the square shape rather that the black circle.

I don't think there's anything wrong with aiming at a square shape when using a reticle full of hashmarks, but could it be that the mirage had some effect on my aiming? Note: there was no mirage from the sun heating the ground, just from the suppressor getting hot.