HUMIDITY EFFECT ON GROUPS?

samson

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Mar 25, 2003
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243
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PA
I went to the range the other day and shot proven loads with my 300 WM and 6.5 PRC. The loads I shot have been giving me between .4 and .5 5 shot groups. This day was more like 1" + groups. It was 85 and Humid. Would this effect the groupings this much? Also notice on the 6.5 a heavier bolt lift. The 6.5 was shooting 156 Bergers and 55 grns of RL26. The 300 WM was using 72.5 of IMR 7828. Thanks.
 

LaHunter

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Sep 30, 2012
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501
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N.E. Louisiana
I use IMR 7828 in my 7 mm mag. I have found that about 80-85 degrees is where I see the temp has an effect. My velocities increased and my groups opened up similar to yours. Below 80 degrees and it doesn't seem to affect my rounds. I doubt the humidity is the culprit.
RL26 seems to be well documented that temps in the 80-85 degree range will have a significant impact.
If the internet lore is correct, RL26 really pressures up around 80-85 degrees, but I have never used this powder.
Temp sensitivity is a real thing. I keep using the IMR7828 because it works so well in my rifle in normal hunting temps.
 

samson

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Mar 25, 2003
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PA
Ok, that seems to be the problem. I will try on a cooler day and see what happens. Thanks for the info
 

new2mud

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May 3, 2010
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Heart of TX
Did you chrono the loads at 85F? I would imagine they ran a bit faster than your prior smaller groups given they are hitting the “temperature/pressure wall” of RL26 around that point.
 

Flintlock shooter

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May 2, 2018
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Spokane WA
You may want to look for a accurate charge weight a little below the loads you're running now. I'm guessing you won't give up much speed and your problem will go away. It would also be easier on your brass.
 

Pete H

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Sep 28, 2019
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Arizona
Temp and humidity could have some affect, I dont think that it makes that much of a difference at a 100 yds. but I find that not every day you go shoot will be the same. You might have something on your mind, it might just be an uncomfortable day??? But I shoot 3-4 times a week and once in a while, I cant group worth a darn. So I pack it up and try another day. Just my 2 cents...
 

IHFarmer07

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Dec 29, 2013
Messages
195
I went to the range the other day and shot proven loads with my 300 WM and 6.5 PRC. The loads I shot have been giving me between .4 and .5 5 shot groups. This day was more like 1" + groups. It was 85 and Humid. Would this effect the groupings this much? Also notice on the 6.5 a heavier bolt lift. The 6.5 was shooting 156 Bergers and 55 grns of RL26. The 300 WM was using 72.5 of IMR 7828. Thanks.
How long were the loads “loaded”? You might have had a little bit of cold weld going on. Otherwise yes, weather would/could affect these things.
Also, was the barrel clean, dirty? How many rnds down the barrel, 100, 200? Barrel may need to be cleaned otherwise.
 

samson

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Mar 25, 2003
Messages
243
Location
PA
The barrel had about 20 rounds thru it after it’s cleaning and the bullets were just loaded. I’m thinking they may just be a bit too hot of a load. Will back down a bit and see. PETE H, maybe it’s like my golf game. Sometimes it’s just not there! 😉
 

Slibey

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Feb 23, 2020
Messages
30
Location
Idaho
Not knowing a lot, I would think that high humidity would make the air denser and change velocity, as high temps effect velocities. Heating cartridges in the sun or high heat can change pressures and velocities. I read the Canadian sniper that held the record for awhile, ran out of his loads and got some American ammo, laid em in the sun to gain velocity and worked, don’t how factual but kinda makes sence
 

Tulsa Reiner

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Jan 6, 2014
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216
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Tulsa, OK
The barrel had about 20 rounds thru it after it’s cleaning and the bullets were just loaded. I’m thinking they may just be a bit too hot of a load. Will back down a bit and see. PETE H, maybe it’s like my golf game. Sometimes it’s just not there! 😉
Samson: I blame the balls on those days when my golf game is off!
 

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