Velocity Change do to Temperature

[lht645]

I am using the following load in a 22", Stiller Tac-30 in 308. I got 2638 FPS at 87 degrees in the summer and in the winter (for SC) I'm getting 2575 at 55 degrees. Any thoughts on if this is reasonable for temperature based velocity change using Varget ? It was more than I expected ? Strings of 5, 12 SD, clocked with a Magneto-speed.

Nosler Brass
Nosler 175 RDF (308)
42.5 Gr Varget
FGMM primers

 

[canezach]

I would suspect lot to lot variance more than just temp sensitivity, unless you loaded those rounds in summertime and kept them until winter. Precision Rifle Blog (Yeah, I know...) tested a few powders and Varget velocities only swung 46 fps over a 115º swing, and most of that velocity change was an increase in velocity from their ambient temp of 65º (2693 fps) to 140º (2728 fps). From ambient to 25º, they only saw an 11 fps loss.

 

[culater]

Temperature change will always effect your powder charge and speed now matter what powder you use. I use Varget in my 308 to and get about 50 fps or more from summer to winter.

This is what I do to deal with this issue. I would make a record of the 4 seasons you shoot in by going out an recording the speed and shoot at a 300 yd paper target. If you know your dope card is in the summer time at 300 yds. Then go out this week end and shoot a 5 rd to 10 rd group at the 300 yd paper target. You should be low at 300 yd during the winter. Then mesure the average group size on how many inches low you are compared to you summer dope. Use a ballistic program to help you find you new zero. Lets say your zero at 200 yd in the summer then it would mean your zero in the winter would be lets say 168 yd zero. Don't re zero anything just print a new dope card and laminated it. That means you'll have 4 dope cards for year around shooting.

The other way is work up loads to the season your shooting in. That would mean you will have to up your charges during the winter. I do is my first suggestion above.

 

[lht645]

Thanks Folks, very helpful, as always.

 

[Herstal 12]

I get around half a mil of elevation change from season to season, no big deal and a log book is great to keep track.
Cheers

 

[Jack's Dad]

Besides temp sensitivity variation the seasonal atmospheric differences will create additional inconsistencies.