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Altitude vs. Barometric pressure

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  #1  
Unread 06-19-2005, 08:50 AM
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Altitude vs. Barometric pressure

Example: RSI Ballistic calculator calculates that at sea level in 59 degree air and 29.53 BP a bullet with a .540 BC at 2700 FPS zeroed at 300 yards will drop 307.9" at 1000 yards.

The same program, same BP, bullet, velocity, zero, temprature, but change the altitude to 5000' and the bullet drop is calculated to be 315.8 with the same barometric pressure imput.

The differance is 7.9"

I always thought that the actual pressure and not altitude changed the bullets perfomance. I know pressure changes with higher/lower altitudes go hand in hand, but thought that altitude in and of itself had no effect on the bullet and just the pressure change with the altitude.

Any ideas?? Is the RSI program correct or is it a glitch?
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  •   #2  
    Unread 06-19-2005, 08:56 AM
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    Re: Altitude vs. Barometric pressure

    What really effects bullet flight is station pressure,(absolute pressure) at your location.

    As an example, a station pressure change from 28.85 to 28.80 means that at 1000 yards my 308 load will need 1 full moa less elevation to strike the same point.

    Different programs try to back out corrected sea level based on temp and altitude. Some hit, most don't.
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    Unread 06-19-2005, 11:42 AM
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    Re: Altitude vs. Barometric pressure

    So if I understand correctly, the most accurate way to use RSI is to leave the altitude field at 0' and just use the pressure field to imput the current pressure at my location no matter what the altitude.

    Right?

    Thanks for the reply!!
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    Unread 06-19-2005, 01:57 PM
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    Re: Altitude vs. Barometric pressure

    If absolute pressure, yes. The confusion comes from "corrected" baro readings for elevation. Say, when the weatherman in Denver says the baro is 29.9, it isn't really absolutely. He's correcting for the elevation, taking the lower reading (24.7 or so) and giving you an equivalent for what it "would be" at sealevel. They do this because the corrected number is more important and useful as far as weather predicting goes. Otherwise it would look like every mountain had its own "low pressure system" surrounding it.

    So in short, if you obtain corrected baro numbers for your altitude (29 or something at 5000 ft) you should also enter the elevation into the program. If you obtain absolute baro numbers (much lower for higher altitudes) and use those, you can leave the altitude as zero in the program as the pressure takes care of it.

    I hope that makes some sense. I'm not familiar with this particular program but just speaking in general. With any program you sort of have to figure out what input it's looking for so it corrects, but doesn't double correct.
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    Unread 06-19-2005, 05:30 PM
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    Re: Altitude vs. Barometric pressure

    Couple pretty good articles on the subject here: http://www.riflebarrels.com/articles...erformance.htm

    And here: http://www.riflebarrels.com/articles...e_humidity.htm

    Helped me get a handle on it.

    marc
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      #6  
    Unread 06-19-2005, 07:18 PM
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    Re: Altitude vs. Barometric pressure

    Your program is assuming pressure at altitude of 29.53, instead of 24.923(std conditions at altitude). Nothing wrong with this if you measured and appropriately entered 29.53 at 5000'. Higher pressure, more drop.
    When you enter 29.53 @ 0', this is your pressure at entered altitude. If thats your measured absolute pressure, then your right, you should leave altitude at zero. Even if your standing 5000' up.
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      #7  
    Unread 06-20-2005, 12:13 AM
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    Re: Altitude vs. Barometric pressure

    That all helps ALAOT!!! Thankx guys. I am actually finishing up a ballistic calculator incorporated into The Reloader's Archive program.

    What I have done is inserted two check boxes. One for Altitude and one for pressure. It allows users to choose between which they want to use, but allows only one. When altitude is selected, the pressure field dissapears and is defaulted to 29.92. When pressure is selected, altitude dissapears and is defaulted to 0'. That way the calcs dont get confused. I needed to verify that this was correct before I finish it up and start distributing updated copies.

    Thanks again! [img]/ubbthreads/images/graemlins/smile.gif[/img]
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    Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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