Powder thermal sensitivity

[paul scrobohaci]

In a nutshell, I wanted to see how muzzle velocity is affected by powder temperature for Vihtavuori N565 powder.
I found that MV velocity goes up about 0.33 FPS per degree F for a 300 Win Mag round containing 75.2 G of powder with a 168 G TTSX bullet, CCI large magnum #250 primer, Lapua brass, shot out of a 26'' barrel.

I think that 0.33 FPS per degree F indicates very small sensitivity.
On an average day where temp varies by 50F, MV won't vary by much more than +/- 8 FPS which is less than the ES most guys get consistently.

Details in my next post.

 

[paul scrobohaci]

Below is the measurement data graph.
MV was measured with a labradar.

Blue dots/lines are for measured data.
The confidence intervals correspond to the extreme spread in measurement for both MV and temp.
I measured the temp with a thermocouple directly on the brass before chambering it.
I kept the rounds in metallic boxes inside the heating/cooling units.

The red line is my eye-balled regression through the data which indicates a slope of ~0.33 FPS / (deg-F).

The numbers of measured shots for each datapoint starting from low temp were (4, 4, 2, 4, 5, 4, 4)

 

[paul scrobohaci]

In a slightly different vein: I experimented with using non-magnum large rifle CCI primers (clearly not what 300 WinMag calls for) and found that I can systematically shave ~10 fps off the MV ES (presumably because of the smaller disturbance they cause) and never had any misfires (over 30-50 rounds). The average MV did go down ~60 FPS by doing this.

I took advantage of this experiment to find out whether I'm at the low limit of the non-magnum primer's ability to ignite the powder, by cooling rounds with non-magnum primers to 25F. All 3 (only) fired fine. And yes, it's not a statistically significant sample, I'll have to try it again with at least 20.

It is conceivable that this practice may cause faster barrel throat erosion because of less complete powder ignition.