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affect of ambient temp on velocity?

 
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Old 05-15-2013, 03:58 PM
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Re: affect of ambient temp on velocity?

Quote:
Originally Posted by X-man View Post
Changes in powder temperature quite often move POI horizontally as well as vertically. This is why experienced shooters keep a shot data log book. My 300Win moves about 1/2 MOA left for a 30 degree increase in temperature.
X-man,

Great reply with noted experience. As you pointed out, the internal ballistics due to velocity changes can be significant.

Many times during load development I have adjusted the powder charge by .2 grains. Group ES with the different powder charges and group size may be similar however the POI between the two groups sometimes have more of a horizontal correction as much as vertical due to the change in velocity and other factors causing the changed behavior to the internal ballistics of the gun. Good comment addressing powder temp, velocity data regression and internal ballistics all being interrelated. If not correcting for the difference in temp/velocity as you have noted, keeping your powder temp while hunting as close to your zero temp, assuming your powder temp has adjusted to the outside temperature during zero, and using a less temperature sensitive powder such as Hodgdon Extreme can be an alternative.

One simple technique to compensate for significantly colder field temperatures than your zero temp is keeping your ammo or magazine in an outside breast pocket. Your body temperature will warm the ammo or magazine increasing the powder temp above outside temperatures. You can also place your Kestrel inside the same pocket to verify the powder (pocket) temp.
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