Air density/temp?


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hancjamk
December 7, 2010, 02:25 PM
This one got me a little stumped. Went to range yesterday and the outside temp was about 20 degrees less than my trip the week before. I noticed my groupings had opened up bit, with an occasional flyer. The only thing I can figure it must have something to do with air density. Can a 20 degree difference really have that much of an effect?

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1858
December 8, 2010, 12:54 AM
This one got me a little stumped. Went to range yesterday and the outside temp was about 20 degrees less than my trip the week before. I noticed my groupings had opened up bit, with an occasional flyer. The only thing I can figure it must have something to do with air density. Can a 20 degree difference really have that much of an effect?

This is where a chronograph and weather meter are helpful. A chronograph would tell you how much slower your MV was at T2 = T1 - 20˚F, and a weather meter (with DA output) would allow you to compare density altitude. Without those, you're just guessing.

Were you comfortable when shooting with the temperature 20˚F colder? Were you shivering? Was the average group center in the same place on the target compared to groups shot under warmer conditions? If you consider that each bullet is exposed to the same air density, if the velocity is the same (similar) and the barrel is pointing in the same place, your groups should be the same, although the POI may be off due to the change in MV. So if your groups opened up considerably, or worse, they opened up and the average group center was off, then it comes down to either inconsistent MV or barrel harmonics, or both. The optimal charge weight (OCW) method tries to address this issue. The idea is that the OCW load isn't pressure sensitive i.e. +/- 0.3gr (for example) either side of the OCW load results in the same position of the average group center for all three loads. The groups to either side of the OCW load may be slightly larger but not by much. Cartridge, chamber and barrel pressure will be affected by ambient temperature ... the question is by how much. If the velocity is reduced or increased significantly due to ambient temperature, this is the equivalent of having a different amount of powder in the cartridge case thereby changing your accurate load at X˚F to an inaccurate load at X˚F + Δ˚F.

I haven't seen any data for change in MV as a function of temperature. Most people would have to obtain that data empirically i.e. work up accurate loads at different temperatures. This would make a good discussion for those that shoot all year round in states that have four very different seasons.

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