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How Much Does Barometric Pressure Change?

 
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  #1  
Old 08-02-2008, 08:06 AM
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How Much Does Barometric Pressure Change?

I have paid little attention to pressure as I usually have shot game at distances under 700 yards with a very flat shooting load. However, I am now shooting with a handgun and a much less flat shooting load.

29.53 is the default used by my Oehler Ballistic Explorer software. Two inches of pressure difference (to 27.53) change my POI by 2.8 inches at 700 yards. In your real world experience, how much variance can there typically be above or below this 29.53 default reading? How common would a 2 inch pressure change be?
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Old 08-02-2008, 08:23 AM
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Len,
Here in eastern wv, I see swings of 3" pretty often, which at 1K will change POI 36" or close to that.
RR
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Old 08-02-2008, 09:01 AM
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RR

That is a huge POI change, even for 3 inches of pressure difference. What BC, V, are you using to see that much change?
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Old 08-02-2008, 10:11 AM
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Len, thats just an example I use, I ran the numbers once I think it was my 270 wea. firing a 140 gr GKHP @3200 fps, and a 4" swing in BP changed the POI like 50" at 1K, think the BC is .354.
from 29" to 31.5" of BP swing the BC runs from .357 to .329 drops will change POI at 1K from -318.5" to -349.5. 700 yard drops BP of 29.4 -99.06, 31.5"- -105.81 so that 6" of drop at 700 computes to 41" at 1K
like stated before an example I use for the shepaerd scope and other BDC guys who make those outlandish claims about 1K shooting with a BDC.

Just reran the numbers again using load from a disc for windows
RR
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Last edited by Ridge Runner; 08-02-2008 at 10:29 AM. Reason: ran the #'s
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Old 08-02-2008, 10:28 AM
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Milwaukee monthly highs and lows. Of course there is so much beer involved that it is hard to beleive the data.

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Old 08-02-2008, 10:42 AM
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RR, thanks, yes, those low BC's make accurate pressure readings important, don't they.

BB, exactly what I needed, thanks. So, the biggest yearly baro spread was about 1.8 inches. That's what I was wondering. So with my current handgun load, POI could shift 2.8 inches at 700 yards. (only 1.4 inches with my former load/rifle combo)
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Old 08-02-2008, 11:13 AM
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Len the real problem is when you change locations. If I shoot at my local range the pressure will run 27-28, if I go to my hunting area it runs 23-25. If you are shooting BC challenged bullets or extreme distances it makes a big difference. In the end it is one of the many things to add to the compounding errors list. A little off on BAR, a little off in elevation, cosine of .94 instead of .96, then add in some wind components and scratch your head why you missed.
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