temperature changes

[j_unzicker]

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.

 

[Codyg510]

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.

 

[Joe King]

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

 

[4xforfun]

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.

 

[j_unzicker]

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.

 

[moombaskier]

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.

 

[j_unzicker]

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.