Warm vs. cold Velocity's

[Bigwheels]

Kind long, but bear with me.
I just did an experiment with my 300 WM & H-Retumbo.
Some may remember that I asked a question about cold velocity's a while ago, as my pet load of 78gr H-Retumbo with 208 A-Max that usually gives 2911 fps had changed. That load was tested in 50* temps.
During the winter I was seeing pressure signs, & re-chrony'd @ 3000fps+. So I backed the charge down to 77.6gr, & chrony'd @ 2960fps. That was in 40* temps.
The last couple times out I was shooting lower than expected, & I chrony'd again, & now in 60* temps, I'm averaging only 2830fps.
Coul the Retumbo be that temp sensitive, the wrong way? Or is it more likely that something else is causing this?
After doing the math I'm showing a -6fps/*F difference in powder temp.
Rifle is a Montana Rifle 26", 1 in 10" tw, #8 contour SS, no flutes.

 

[rmfield]

Re: Warm vs. cold Velocity's

The short answer to your question is that it is very unlikely that the Retumbo is "reverse" temperature sensitive. Look at your sample size, barrel heating and powder charge variation.

Edit: There are several factors that could be affecting your chrono data as well - distance to screens, light variations, etc.

 

[Bigwheels]

Re: Warm vs. cold Velocity's

I was very careful to keep the chrony's the same distance from the muzzle. I used 2 chrony's back to back, & their results were only 0-10 fps different from one an other. Sample size was at least 40 rnds for each test. Some were tested on different days under similar conditions, & showed the same results.
Results were also consistent for elevation to a given distance. Shot flatter when cold, than when warm. 15.5 moa to 775 yds @ 40* F. 16.5 moa to 775 yds @ 65* F. Baro. Press was only 1.3" difference between the tests.
My only other variable is the powder lot's are different. I guess that might be it.

 

[ChadTRG42]

Re: Warm vs. cold Velocity's

Plus, you have to look at the actual weather conditions. The barametric pressure, humidity, change in zero, and other factors can make this difference. I have also shot through a chrono and it be off due to the sun light angle hitting the sensors.
Throughout a few days of shooting, I have had my zero change, and no expaination why. You just have to follow what your rifle is telling you at that time. The only other thing remaining is FM! (Freakin' Magic)

 

[Bigwheels]

Re: Warm vs. cold Velocity's

Well, I guess it must be FM. I always carefuly set up my chrony's with the shades on, & a tripod with tee shirts or something to block the sensors from direct sunlight.
I think I'm going to load some more, & keep some in a cooler, & some not, & start my shots with the same barrel temp, etc. & see if that will replicate what I saw during the last couple of months. Sometimes this "Shooting Stuff" can be so frustrating!
I may buy some H-1000 & try that too.