Powder charge vs temp fluctuation and accuracy

[HairyScandinavian]

So I found a decent load for my 6 ARC AR15, 29gr of Lever shoots consistently better than 29.5gr with 95 TMK's. I'm happy with it. So it's been 80's and 90's here while testing so speeds are elevated with Lever being pretty temp sensitive at the higher end of temps. My question for those of you with more experience than me is when the temperatures drop and the 29.5gr slows down to the speeds I'm getting with the 29gr now, will the 29.5gr inherit the 29gr's accuracy with the slower speeds or will the 29gr stay the better charge going slower than it is now?

Please don't tell me I'll have to try it or it'll be February before I know.

 

[Taudisio]

So I found a decent load for my 6 ARC AR15, 29gr of Lever shoots consistently better than 29.5gr with 95 TMK's. I'm happy with it. So it's been 80's and 90's here while testing so speeds are elevated with Lever being pretty temp sensitive at the higher end of temps. My question for those of you with more experience than me is when the temperatures drop and the 29.5gr slows down to the speeds I'm getting with the 29gr now, will the 29.5gr inherit the 29gr's accuracy with the slower speeds or will the 29gr stay the better charge going slower than it is now?

Please don't tell me I'll have to try it or it'll be February before I know.

Possibly.
The bullet is leaving the barrel at precise timing with the barrel vibrations to give you the best accuracy and that could be your “accuracy node”. You should test it.

I did a test with my arc and lever. I posted it on another thread that I can’t remember, but you can easily replicate it with your loads and test for yourself.

I loaded up 10 identical rounds. Shot a 5 round group with ones that were at ambient temperature. The second 5 I left in the freezer two days prior, pulled the ziplock of rounds out, filled another ziplock with ice and stuffed the one with the 5 rounds inside like a little cooler. 10 minutes up the mountain. Shot back to back groups.
The frozen group shot high right and opened up. It’s late and I don’t have my notebook, but it was like 55 or 60 degrees out, and the velocity change was less than I expected. I assume the frozen rounds were around 20-30 degrees and the difference was 40fps or so IIRC. 2730ish vs 2690. I’ve seen the same load up past 2750 when it’s warm out. Based on my previous data-2695 was my avg for the charge a half grain lower.
It isn’t perfect science, but I got real, notable, results anyway. Throw 5 rounds of 29.5gr in the freezer and shoot them cold next to your warm temp load. I am very curious and would try to do the same with mine to see if the results match.

 

[Castle Rock]

Most likely you will have to step the load up in the cold.
Velocity nodes are very real.
That is why benchrest shooters load at the range, they tune to the conditions.

 

[The_Yetti]

Unless you can find where someone tested your exact load for temperature sensitivity, you're gonna have to test. Many different factors can effect temperature stability.

 

[Shortschaf]

Stuff some rounds in a cooler with ice packs and test them out. document temp with an IR thermometer.
Wipe off condensation before shooting

 

[Castle Rock]

Unless you can find where someone tested your exact load for temperature sensitivity, you're gonna have to test. Many different factors can effect temperature stability.

Not really, this is all tested and documented, there is even an input box in the shooter app for powder temperature sensitivity

 

[The_Yetti]

Not really, this is all tested and documented, there is even an input box in the shooter app for powder temperature sensitivity

I'd still test it. Especially if it's something that's on your mind it will be until you do. It's also been documented that temperature sensitivity can vary based on application.

 

[Shortschaf]

Not really, this is all tested and documented, there is even an input box in the shooter app for powder temperature sensitivity

Are you saying there is a reliable database that lists the fps/°F for each powder?

Or just saying that, generally, colder=slower? Because I agree with this.

If someone wants to know what the speed and accuracy is between 0 and 100°F I don't know of a reliable way to predict that instead of testing it

 

[HairyScandinavian]

I hadn't thought of throwing them in the freezer/cooler. I've got about 30 rounds of the 29.5gr still loaded up so easy to try that out. I know it'll be slower I was just pondering if accuracy was mainly based on velocity or more of a function of the charge/powder volume.

 

[RWT]

Watching as I am about to start my Grendel reloading adventure. Hoping I can push 130TMK’s

 

[Shortschaf]

Sharing bro science method below

In my opinion, it is not temp stable at all. Roughly 1.8fps per degree Fahrenheit.

Loads below are 59.0gr N560 in 6.5 Sherman Short 20" barrel with 135 classic hunter:

3 rounds in ice bath (34°F) yielded:
2821
2818
2835

3 rounds that were baked on a pickup dash with the windshield defrost on max (102°F):
2958
2948
2946

So roughly 130fps increase over 68°F increase

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