Temp change effecting load?

Hawkeye2005

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Stumped, was curious about temperature change coinciding with powder load and group size. Never fails, spring time I get my powder load shooting a 1/2" and as the summer wears and temp goes up so do my group sizes. I've gotten to the point where I only shoot three shot groups thinking that the barrel is getting to hot or the mirage off the barrel is screwing with me. But then I got to wondering if a 20-30 degree difference is what is effecting the group size (Iowa aka hot and humid), driving me nuts to the point I start testing loads again. Been shooting and reloading every week for the past five years now and still learning (at a slow pace mind you, hahaha). Looking for some pointers.
 

ofcrfs242

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Re: Temp change effecting load?

Gonna need more information. What caliber load, and powder. Some powders are affected by temperature swings quite significantly while others such as Varget temps seem to make no difference.

Have you seen any difference shooting during the winter or at colder temperatures.
 

Hawkeye2005

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Re: Temp change effecting load?

Calibers:
338LM - Viht 165 250gr Accubond struggling for consistency
30-06 - IMR-4064 150gr Hornady SST
308 Win - Just started with Varget and Berger 168gr hunting have yet to finalize load, using RE15 prior to that.
7mm Rem Mag - Viht 165 140gr Accubond
270 Win - H4831sc 130gr Accubond
243 Win - H4831sc 95gr Nosler BT Rem700 VLS, really getting close to buying a custom pipe bender for this one.
 

Dasher

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Re: Temp change effecting load?

The hotter the tempeture gets some powders produce more pressure with the same powder charge. When the tempeture is hotter try reducing your load a little.
 

Hawkeye2005

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Re: Temp change effecting load?

How much reduction in powder load does it take to get the accuracy back to where it was...0.2, 0.5, 1.0gr???
 

longebow

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Re: Temp change effecting load?

Only way to know is to test it in the conditions that you are shooting in. Agree with the concept of picking a "temperature insensitive" powder. I had TERRIBLE velocity/pressure changes with Win 748.

Don't forget the contribution of HUMIDITY, which you already alluded to. Increasing humidity will also increase pressures with any given load. There are Kestrels that will measure both for you, or you can just keep track of your environmentals by checking the local weather station.
 

Lindy

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Re: Temp change effecting load?

I've never seen any data on the effect of humidity on chamber pressure.

Since powder contains it's own oxidizer, and you can fire a round under water, I'm inclined to think there isn't one. If you have some data showing such an effect, please cite it.