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Temperature and humidity factor - how do they affect my loads?

 
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Old 07-18-2006, 01:52 PM
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Re: Temperature and humidity factor - how do they affect my loads?

So, what I now deduce is that temperature affects both external ballistics and powder behaviour. Just a few days ago, it was 25C (77F) here in the evening. I shot some loads of 270WSm using Hornady 140gr, behind 59gr of IMR 4831. Grouping was 1.25" and velocity averaged 3175 fps. Yet the IMR manual states a velocity of 3081 fps for a 60.5 gr load of this same powder. My loads clocked nearly 100 fps faster with 1.5 grain less powder. This must be the obvious results of this temperature differential that we speak of. One would seemingly be quite impressed about the gain in velocity, however there must also be a gain in pressure which could be a concern, although my spent casings showed no such signs. I think that from what I'm reading is that likely the idea to keep the loads and barrels as close to hunting season conditions as possible is likely the way to go. Keep in mind, I frequent this site for the great deal of reloading information to be learned, but I am by no means a "real" long range hunter. I shoot big game to 400-500 yds, that's it. I'll likely be huntig anteloppe in September when it's still pretty warm and need to work up my loads now, not a week before season opens.
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Old 07-18-2006, 02:35 PM
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Re: Temperature and humidity factor - how do they affect my loads?

[ QUOTE ]
Grouping was 1.25" and velocity averaged 3175 fps. Yet the IMR manual states a velocity of 3081 fps for a 60.5 gr load of this same powder. My loads clocked nearly 100 fps faster with 1.5 grain less powder. This must be the obvious results of this temperature differential that we speak of.

[/ QUOTE ]I don't think it's local atmospheric conditions. You probably used a different barrel as well as powder lot and maybe a different bullet, primer and case. All of which can easily result in a 100 fps difference from published load data even in identical atmospheric conditions. Plus two people can easily get more than a 50 fps difference shooting the same ammo and rifle from bags atop a bench just because they hold the rifle differently.
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Old 07-19-2006, 06:49 AM
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Re: Temperature and humidity factor - how do they affect my loads?

[ QUOTE ]
Plus two people can easily get more than a 50 fps difference shooting the same ammo and rifle from bags atop a bench just because they hold the rifle differently.


[/ QUOTE ]

I must admit that I didn't think of that, but how do the mechanics or physics of holding a rifle differently than another actually work? - interesting though.
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Old 07-19-2006, 08:49 AM
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Re: Temperature and humidity factor - how do they affect my loads?

[ QUOTE ]
...how do the mechanics or physics of holding a rifle differently than another actually work?

[/ QUOTE ]It's caused by how hard the rifle's held against the shoulder. A hard hold reduces the amount of rearward recoil which lets more energy push the bullet out the barrel resulting in higher muzzle velocity. You'll get higher velocity with the rifle's solidly fixed in place (butt against a tree?) than fired unheld in totally free recoil.
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Old 07-19-2006, 10:25 PM
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Re: Temperature and humidity factor - how do they affect my loads?

Again, I hadn't thaught of that, but it makes perfect sense. Thanks Bart_B and all others for your valuable input.
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Old 07-21-2006, 09:06 PM
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Re: Temperature and humidity factor - how do they affect my loads?

[ QUOTE ]
I've never seen any accuracy change from 20 to 105 degrees F, 5 to 90 percent humidity and 200 to 8300 feet altitude with the same rifle and ammo

[/ QUOTE ]

So let me get this straight. You have never witnessed a sweet spot or "node" get overpressurized from 90 degree weather and shoot worse groups or open up standard deviations?! Boy, somebody needs to find a sweet spot over a chrono on a cold day and then fire that ammo over a chrono on a July afternoon! Can you say "wow"?

Where do I purchase one of these properly fitted barrel lazer guns? I need one badly. I'm gonna have to call Kirby and complain that my gun barrel won't shoot a good group when the crown is so hot it's setting off my neighbors ammo on the bench next to me! [img]/ubbthreads/images/graemlins/wink.gif[/img]
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Old 07-21-2006, 09:08 PM
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Re: Temperature and humidity factor - how do they affect my loads?

[ QUOTE ]
It's caused by how hard the rifle's held against the shoulder. A hard hold reduces the amount of rearward recoil which lets more energy push the bullet out the barrel resulting in higher muzzle velocity. You'll get higher velocity with the rifle's solidly fixed in place (butt against a tree?) than fired unheld in totally free recoil.

[/ QUOTE ]

Oh, and by the way, PS did an article on that very topic awhile back and found it to make a difference in the order of something like 10 fps or less. [img]/ubbthreads/images/graemlins/shocked.gif[/img]
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