Altitude vs. Barometric pressure

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Michael Eichele, Jun 19, 2005.

  1. scfam

    scfam Active Member

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    BINGO. Absolute baro pressure, along with humidity and temp pretty much has you handled. If you are using relative, then you need to also use elevation. Good answer.
     
  2. Bart B

    Bart B Well-Known Member

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    This is the first time I've heard of any gravity issue.

    Never in my thousands of talks with long range rifle shooters at matches and social events with many of them has this issue came up regarding altitudes of 100 to 6,600 feet above sea level shooting 1000 yard matches. Nor has that of coriolis and spin drift. But such things appear on web sites very often and claimed to be very important.

    Exactly how much difference in bullet drop at 1000 is there for a 30 caliber 200 grain bullet with a BC of .560 leaving at 3000 fps at 200 feet and 6600 feet elevation in standard atmospheric conditdions?

    Is it greater than a 1% spread in that bullet's BC would cause?
     
  3. Mikecr

    Mikecr Well-Known Member

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    Gravity is difficult to predict. It changes with geographic density (like mountains) regardless of altitude. Dismissing this, your gravity difference from 6400 feet scenario could lead to .30" change at 1kyd. 1% BC would be 1.2".
    So gravity, and given it's unpredictability, is really too small an affect to measure or account for with small arm ballistics(even at distance).

    But technically it is a factor, and so is spin drift and vertical and horizontal Coriolis.
     
  4. Bart B

    Bart B Well-Known Member

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    One thing few understand is the wind above the line of sight is blowing faster than in the line of sight. Here's a table showing this and two ammo types to see how it effects wind doping:

    [​IMG]

    And the wind closest to the person shooting has more effect on bullet drift than the wind close to the target.

    [​IMG]
     
  5. alcesgigas

    alcesgigas Well-Known Member

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    Thank you Bart for your comment concerning wind effect: It's a good feeling to have confirmation, scientifically, of what you've reasoned (or logic) is accurate. I've pretty much held that wind effect is greater at the target the closer the interference is to the projectiles origin. My reasoning has been, like a small branch nicked by a bullet at 50' is more likely to miss the target at 300 yards than the same nick--all other things being equal--occurring 50' from the target. It's similar to a .25" miss at 100 yards is most likely to miss by 2.5" at 1000 yards; same for scope adjustments.

    In spite of its span in years this is a most informative thread; I'm enjoying it still.
     
  6. CatShooter

    CatShooter Well-Known Member

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    When you look at the "External Ballistics" page, there is a "Conditions" tab.
    Use that and choose "Standard conditions (Army or ICAO).

    Then, there are two small diamonds... "Station" and "Altimeter".
    Click "Station".

    Then enter your actual barometric pressure from your hand held instrument (not from the radio weather report - because it is "adjusted", and not actual, local pressure).
     
  7. LastShot300

    LastShot300 Well-Known Member

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    Bart

    Do you know, personally or not, if this formula is really accurate? Thanks for sharing it.
     
  8. Hand Skills

    Hand Skills Well-Known Member

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    image.jpg
    Supplemental data
     
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  9. Litehiker

    Litehiker Well-Known Member

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    Just buy a Kestrel/AB 5700 and let it calculate all the variables including wind at the FFP and you will have a solution that includes "station pressure" and even coriolis effect if shooting beyond 900 yards.

    Eric B.
     
  10. Trnelson

    Trnelson Well-Known Member

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  11. RTK

    RTK Well-Known Member

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    Amazing how many fellow pilots hunt.
     
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  12. corey006

    corey006 Well-Known Member

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    So just a quick question.

    You arrive at a hunting location.

    Zero your rifle for that elevation and temps are relatively stable.

    How does day to day changes barometric pressure affect POI @ 700+ yards IF elevation and temp remain constant?
     
  13. CatShooter

    CatShooter Well-Known Member

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    If all else stays the same - pressure goes up, bullet goes down.
     
  14. wildcat westerner

    wildcat westerner Well-Known Member

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    Will try to add .02 worth to this discussion. I am way behind the curve as to ballistics programs that can provide the CORRECT answer for a single shot at long distances. Coming from sea level Seattle to 6,500 feet in the Rockies I note my 6.5 x 284, which I have been shooting for 16 years, shoots flatter to the point you can tell on a target at 300 yards. So your statements as to entering 0 for altitude in your programs, really takes some adjustment on my part. I do understand you are speaking of arriving ata right answer and this is the best way to manipulate a ballistics program to achieve a correct conclusion. I have never used my I phone in the field to use a ballistics program, and realize I will also have to carry my anenomoter along also to access the correct data for that location and weather information. Bottom line to me is this: I have been told by a very savvy shooter that if you compare between free programs and very expensive ballistics programs, the actual difference between accuracy at long range is very little. The bottom line for me is which is the very best program i want for my I phone, for the money?
    Thanks for your attention in this matter.
    Gene S.