Cold weather presure

[patchhound]

Will cold weather change presure levels? I know heat will cuase exess presure but what is the afect of lets say +17 or -3F. Will find out when CZ`s done, I don`t want to be in the field to learn though. MY load is 6br 31.8 H4895 87gr Vmax cci450pmr. 32. starts to get warm.

I`ll have to retest my loads with my new chamber but not when its -3f. Any thoughts?

 

[Forum Boss]

Yes temperature will affect the pressure of a loaded round, and the differences can be significant, particularly if you're near max.

Asa Yam raised a very important point that pressure rise/fall in extremely low temps "depends upon the powder". At the link he cites, in a 30-06, AA4350 actually shows quite a bit more pressure at 0 deg F, than a 70 deg F. That said, more commonly you'll see lower pressure and lower velocities.

QuickLOAD has a bare-bones temperature adjustment feature. I ran your load at +17 F, and at -3 F, with a 26" bbl, 37.8 grains H20 capacity,the default), .220" of shank,bearing surface) in the neck, no land contact with bullet. Mind you I don't have your exact case capacity or seating depth, so the results are only a very loose estimate:

Pressure at +17 F = 62,730 psi

Pressure at -3 F = 60,895 psi

With 38.2 grains H20 capacity, the numbers are:

Pressure at +17 F = 61,124 psi

Pressure at -3 F = 59,341 psi

The other thing to keep in mind is that, at very low tempartures, the barrel chamber will shrink slightly, and the primers may not behave normally. This can also affect pressure.

 

[AndyW]

When I shoot in the winter I average 100 FPS less velocity than in the summer. This is why I do all of my load development in the summer. Working up a hot load at -15c and then shooting on a hot summer day could be a big problem.

 

[Asa_Yam]

Patchhound,

It depends on the powder. For most powders, burn rate increases with temperature. A few show pressure increases in cooler temperatures as well. See http://www.hodgdon.com/smokeless/extreme/page3.php#308win for examples of powder behavior with temperature.

Near-max loads will show less tolerance to pressure extremes, and hence temperature extremes.

 

[Forum Boss]

Asa,

Thanks for your post. You raised a very important point. Some powders, acc. to the Hodgdon chart you cited, actually pick up pressure at very low temps.

I will say also, I think one needs to do one's own testing. Varget is supposed to be very temp INsensitive acc. to the chart. However, I observed and chron'd a 55-60 fps variance in velocity between 6BR ammo that was 80 deg F, and the same ammo that had been sitting in the sun and was warm to the touch--probably about 110 degrees. All cases loaded with identical loads of Varget at the same time.

The 110 deg ammo gave bright ejector marks and the 80 deg ammo did not. Working with QuickLOAD, extrapolating backwards from the observed Vel differences, the 110 deg Varget load was running about 4000 psi greater than the 80 deg load.

Allowing my 6BR ammo to rise from 80 to 110 degrees was like adding .7 grains of Varget in terms of pressure. Acc. to Hodgdon, the change in temp shouldn't have made a significant difference. But it sure did. Big time.

That's why I now view manufacturers' claims of temperature insensitivity with some skepticism.

 

[USAPatriot]

I agree with that, Mod. I always had problems with Varget over about 85 degrees,and at 6500-7000 feet elevation) in my .223 VS. It was great low temp shooting and I often shot it when it was snowing without problems. Fliers, cratered primers, outgassing and other problems with Varget just became too common in the summer. I considered packing the rounds in a cooler with a non-ice cold pack as an experiment but never got to it. -Rod-

 

[patchhound]

Thanks everybody for the info.

I`ll get my CZ 6br back in mid Jan. so I`ll find out what cold does. Since I only use H4895 and varget I`ll have less results too share but I`ll post what I find out then. I never had any problems with either loads of 31.8 in the summer but like alway`s, that`s just my gun.