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Can Outside Temperature Affect Cartridge Velocity?

If you zero your rifle in the fall, then shoot prarie dogs in the summer, will your rifle shoot to the same point-of-impact?

Can Temperature Affect Bullet Velocity and Point-of-Impact?

By Jeff Johnston

The Myth: Outside temperature does not affect the velocity of rifle cartridges, therefore point-of-impact remains the same regardless of temperatures.

The Expert Deferral: On page 241 of the NRA Fact Book, Mr. E. H. Harrison writes: "Temperature has a marked effect on both velocity and pressure. ... Especially does temperature affect pressure. ... The matter of temperature effect on velocity is simpler. For the IMR powders, each change of temperature of one degree Farenheit changes the muzzle velocity by 1.7 fps in the same direction. Thus, and increase in the temperature of the powder amounting to 20 Farenheit could be expected to increase the MV by 34 fps.

... It should be borne in mind that ammo exposed to the direct rays of the summer sun will reach a temperature much higher than that of the surrounding air. A temp within the ammunition of 130 Farenheit or even considerably more would not be unusual under such conditions. ... Even worse is the condition within the baggage compartment of an automobile, especially on of dark color, exposed to the desert sun of one of our southwestern states.

... It can be taken as a general rule that factory ammo and moderate handloads will give no trouble due to temperature effects, if the ammo container is kept shielded from the direct rays of the sun."

The Practical Meaning: If you sight in your .243 Winchester before deer season in 40-degree weather then go prairie dog hunting in the 100-degree summer heat where your cartridges are exposed to the sun (reaching 130 degrees inside the cartridge case), expect an increase of around 136 fps.

In a .243 Winchester 100-grain load that has a MV of 2850, expect a point-of-impact shift of -2.5 inches at 300 yards (From 14 inches low at 300 yards with a 100-yard zero to 11.5 inches low in the heat) when shooting at that prairie dog. *Note that an outside temperature of 100 degrees also plays a small role on velocity in terms of atmospheric pressure and that is factored above. 

The Conclusion: The myth is busted. Extreme temperature changes to ammo DO effect velocity and therefore point-of-impact. But for reasonable ranges at moderate temperatures, hunters shouldn't be overly concerned. 

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