Temperature shifts.

[xdeano]

Hay guys I just wanted to run this by you to see if it's my powder or my Chronograph.

155g Lapua, Lapua brass, 210M, 45.4g Varget

My 66F load is running 2825fps using Varget

I went out and shot this afternoon the temp was -7F and felt like -20F. The same load through the chrono was measuring 2564.5fps. I know it was cold but to have such a velocity shift that's crazy.

260.5fps shift over 73 degrees F. Or was it my chronograph? Hodgdon Varget is suppose to be one of their extreme temperature powders and shouldn't have such a huge spread.

Let me know what you guys think.

xdeano

 

[cmillard]

what kind of chronograph--ive had problems before with a shooting chrony in cold weather. but my ced is untested in the cold yet. it could be an issue.

 

[Steve Blair]

Take a look at this post regarding velocity and pressure drop from lower temperature.

Your experience seems in line with mine. The lesson here is to develop loads in the temperatures in which they will be used. I suspect that the reverse is also true. If we develop loads at lower temps, they will be dangerously high at 40+ degrees warmer.

 

[xdeano]

I agree with you on developing a load for time of year, But I figured I'd try and get away it it if I did it at 66F. I didn't figure the temp shift would be this great with Varget. I'm getting an average of 3.56ft/degree which is pretty extreme for an extreme powder. I'm going to have to stretch my legs on this when the wind dies down a bit.

The Chronograph is a chrono beta master, after 4 rounds over it, it started to shoot errors at me like it was getting cold. the screen was going nuts also. I'm guessing it has something to do with the machine and a little bit to do with the powder.

thanks for the link.

xdeano

 

[Steve Blair]

Maybe your Chrony is not all to blame. My result was over 2.8 fps per degree and my Kurzzeit chrono was doing just fine at 55f. I think Varget is only marginally less temperature sensitive than H4895 or otherwise similar powders.

 

[xdeano]

I think what I'm going to do is keep a couple of bullets in my pocket. Let the chrono thaw out over night and take it out again and shoot a few cold rounds and a couple of pocket rounds to see what's going on.

It's funny though, the other day I was shooting at 300 with the same dope and I didn't have to change a thing.

xdeano

 

[cliffy]

Ambient Temperature affects Velocity and Pressure. A notable difference occurs at a mere 20 degree change in temperature. I've yet to find a temperature insensitive powder. Hot is faster and cold is slower, so load for your shooting environment. I Have Michigan loads and Texas loads because no one perfect load for all conditions exists. Cliffy

 

[xdeano]

well I wanted to let you guys know what I've found. I went out again this afternoon.

Recall that my previous chronograph test temp was at -7F and my velocity average was 2564.5fps.

Todays temp was 6F, and my velocity averge was 2811 fps. Which isn't to far off from my 66F zero temp 2825fps.

So my loss due to temperature (temp sensitivity) is 14fps over a 60 degree temperature swing.

That's 1fps change in 4.28 degrees. Now that isn't bad.

This test just goes to show that a Chronograph doesn't work at it's peak at -7F.

I also did ambiant air temp test vs body temp charges.
There wasn't much of a change in velocity. I'll have to play with this some more also.

So varget is pretty stable in my book.

thanks for you time.
Deano

 

[cliffy]

Stable is as stable does, yet a 200 fps deviation regarding a Twenty Degree "shift" is normal in reality. This SHIFT also includes air density and initial muzzle velocity. Cold air is denser than warmed air. Horsepower aside, all firepower is affected. One cannot blame chronographs for inaccuracies regarding ambient temperature fluctuations, since a chrono sees what it sees at the speed of light. Cliffy

 

[RDavies]

Some of the problem might have been in the ambiant light. My crony will show 150 fps faster than actual if it is id day, no cloads. As soon as a cload rolls in (or shade from an umbrella, or later in the day) the velocity reading comes back to reality.

 

[cliffy]

Using the supplied sunshields can make a marked difference as opposed to NOT using the shields. I tend to not use the chronograph shields religiously on cloudy days, but am I wise in not installing the shields? Who knows how much "SUN" actually gets through? Keeping consistancy of "sunblock" will govern my future findings. Cliffy