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temperature changes

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Old 02-02-2013, 12:05 PM
Bronze Member
Join Date: Dec 2010
Posts: 61
temperature changes

Sighted in my 7RM in 40 degree temperature and ran the ballistic and the gun shot true to those ballistics (using territed scope). Went out again in subfreezing temps and at 500 yards shot a 4 shot sub MOA group but it was about 7" low. I am using imr4831 which I think is pretty temperature sensitive, but that seems like an extreme change. BTW - bullets and gun were kept in the the bed of truck for a full day in that subzero weather, so they were consistent with outside temps.
Any thoughts or explanations? Thanks.
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Old 02-02-2013, 12:42 PM
Junior Member
Join Date: Jun 2012
Location: Kansas
Posts: 23
Re: temperature changes

7" isn't likely a temp change in this scenario.

What scope you using? I've had similar issues because my turrets wouldn't dial in a repeatable, consistent manner.
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Old 02-02-2013, 12:50 PM
Platinum Member
Join Date: Apr 2012
Location: The cold part of Montana
Posts: 1,390
Re: temperature changes

One other option you can look at as well is changing to a temp stable powder like H1000, Ramshot Magnum is another that's showing good promise as temp stable
Keep in mind the animals we shoot for food and display are not bullet proof. Contrary to popular belief, they bleed and die just like they did a hundred years ago. Being competent with a given rifle is far more important than impressive ballistics and poor shootability. High velocity misses never put a steak in the freezer.

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Old 02-02-2013, 01:33 PM
Gold Member
Join Date: Oct 2007
Posts: 880
Re: temperature changes

You didn't list what exactly your "sub freezing" temps were. I imputed 40 degrees and 20 degrees. The exterior balistic difference at 500 with a 100 yard zero is very small.

1) Do a chrono test at 40 degrees, or 50, or whatever, and test at lr with your program using your data.

2) Do ANOTHER chrono test at "sub freezing temps"...10 degrees...whatever, imput that data, and do ANOTER LR test using your NEW data.

Let us know what you find.
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Old 02-02-2013, 01:48 PM
Bronze Member
Join Date: Dec 2010
Posts: 61
Re: temperature changes

I'm using a Vortex Viper 6.5-20x50. It seems repeatable. I can run it top to bpttom and back to zero without a change in point of impact. Not 100% sure that it is exactly MOA. Seems like it can be slightly off the ballistic reading for my load and velocity at extended ranges. I should have shot through the chrony in the sub zero weather to see if there was any significant change. Running about 2880 with 168 gr berger, 61 gr IMR4831.
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Old 02-02-2013, 05:08 PM
Silver Member
Join Date: Mar 2007
Location: Dallas,OR
Posts: 345
Re: temperature changes

7" at 500 yds. is way too much drop for just a temp change. That would be some serious velocity change. I could see if the ground was frozen hard, providing you were shooting prone with a bipod, and your shots being higher. I have seen where a Harris bi pod with the leg notch's set at the lowest setting having a bit of spring to the feet causing a low shot/group. If your barrel is not floated enough and making contact with the stock at 40 and not at a lower temp could cause a low shot, but prob not a low group. How about body position or even parallax issues? Somethings awry besides a huge swing in velocity.
7LRM smithed by Gunwerks
Stiller TAC 300
Broughton fluted 5.75, 5c @ 28"
Graphite Black Cerakote
Jewell Trigger
GreyBull Precision Stock
180VLD's@3145fps. with 72.8grs. of H1000, 215m primer
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Old 02-02-2013, 06:31 PM
Bronze Member
Join Date: Dec 2010
Posts: 61
Re: temperature changes

I agree. I think I would have had to have dropped 200 fps to get that kind of drop. Can't see how it could be paralax and still have a tight group unless I did real well at hittting the anchor spot exactly the same with poor paralax. But thought I checked that pretty well. Also same bench rest setup. The exact sub freezing temp (good question by the way) was 5 degrees.
I'll try shotting through the chrony when I find those conditions again.
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