Effects of Temperature on Velocity and Pressure
Velocity and pressure are both measurably affected by temperature.
The following chart was worked out by the famous big game hunter, Les Bowman, of Cody, Wyoming. Les has the following to say about this effect:
"Few handloaders realize the difference temperature of powder makes in velocity and pressure. And the funny thing is that the top temperatures, that is from 72 degrees normal to 100 degrees for instance, affects pressure more than from 70 degrees down to zero. As you will notice in the chart, there is a 129 fps minus for the zero temperature reading at 4000 fps bullet speed, and 155 fps plus for the temperature about 70 to 100. Many sight in one day when it is a certain temperature and later, when temperatures are perhaps 50 degrees different, they think their scope has changed when groups print at a different place. Also maximum loads worked up at, say 40 degrees, can be really dangerous at 90 or 100 degrees. It's the powder temperature that counts, not the temperature of the outside air. I have had a lot of correspondence with DuPont and was told by them that they figure 60 p.s.i. in pressure difference for every degree in powder temperature change. In the case of the 4,000 fps above at 155 on the high side and 129 on the low, or a difference total of 284 fps, that would make 17,000 pounds difference in pressure. This is the correction chart I use, and while I do not claim it is absolutely right, it is close."
Temperature Correction Chart Used by Les Bowman
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