Effects of Heat on Powder Performance
Hodgdon has produced some interesting tables that show how changing temperatures affect the performance of various powders. While most powders generate higher velocities as temperatures rise, it is interesting to note that some powders experience much smaller velocity shifts than others. Here are two charts prepared by Hodgdon:
Extreme Velocity Spread For Temps Between 0°-125°
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Cartridge: 30-06 Case: W-W Primer: WLR 165 gr. Hornady SP |
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Cartridge: 300 Win. Mag. Case: W-W Primer: WLRM 180 gr. Sierra Boat Tail SP |
To learn more about how ambient temperature (and primer choice) affect pressures (and hence velocities) you should read the article “Pressure Factors” by Denton Bramwell. In that article, the author uses a pressure trace instrument to analyze how temperature affects powder performance. Bramwell’s tests yielded some fascinating results.
For example, barrel temperature was a key factor: “Both barrel temperature and powder temperature are important variables, and they are not the same variable. If you fail to take barrel temperature into account while doing pressure testing, your test results will be very significantly affected. The effect of barrel temperature is around 204 PSI per degree F for the Varget load….If you’re not controlling barrel temperature, you about as well might not bother controlling powder temperature, either. In the cases investigated, barrel temperature is a much stronger variable than powder temperature.“
