Updraft question ?

Discussion in 'Long Range Hunting & Shooting' started by Down Under Hunter, Sep 15, 2008.

  1. Down Under Hunter

    Down Under Hunter Well-Known Member

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    Say you are faced with a 750 yard shot into a dead head wind. Closest two thirds of the shot are downhill 20 degree slope then up to the target appox .99 cos. 338 edge. wind speed is about 4 mph hitting you on the nose.

    How much would you knock off for elevation if faced with the situation ?

    DUH
     

  2. trueblue

    trueblue Well-Known Member

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    With the provided information, I am guessing about 2.90 MOA off elevation dial up.
     
    Last edited: Sep 15, 2008

  3. yote bomber

    yote bomber Well-Known Member

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    I'd shoot it dialed for 700 yards...
     
  4. Bravo 4

    Bravo 4 Well-Known Member

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    trueblue,
    Not trying to be a smartallec but where did you come up with that? A 4mph full value wind will only deviate a 300gr SMk (@ 2800fps) 1.5moa off course at 742.5 yards (750 X .99). A 20degree angled up wind would translate into a 1/4 value so that turns the 4 mph wind into a 1 mph wind. A 1mph wind will deviate the 300gr bullet .25moa @ 742 yards, and that is if the wind hits it over the entire range. The updraft the bullet will encounter will only be momentarily when the bullet leaves the muzzle and then it'll be hit with a 4mph zero value head wind. The only elevation change I would make is for the .25moa correction for distance to target(742 not 750). But then again the picture I get from the scenerio is you would be shooting from one hillside/top to another. If that's the case I would be looking for signs of a cross wind in the valley.
     
  5. yote bomber

    yote bomber Well-Known Member

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    So it's not a downhill shot... it's a cross canyon shot at a target that's at nearly equal elevation?? Frickin' story problems... never could figure them out.
     
  6. edge

    edge Well-Known Member

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    Both trains arrive at their respective stations at the same time!

    Also the second man was really a woman doctor :)

    edge.
     
  7. trueblue

    trueblue Well-Known Member

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    Bravo,

    Posted that before my coffee this morning, and worked off dial up and not wind. My revised # is take .60 MOA off dial up. If I am understanding the scenario, you have 500 yards with a 4mph updraft, so at 500 yards w/10mph wind is 1.5 MOA @ 2800 FPS. 1.5 x .4(4mph wind)= .60 MOA.
    Take that amount off your dial up.
     
  8. royinidaho

    royinidaho Writers Guild

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    You've got the situation doped. Scratch the knoggin' a bit and hold a couple of cross hair widths low and let'r rip. I'm assuming you're shooting at a rock here.;)

    If it were an animal, there most probably wouldn't be all of that figgerin' time.:confused: If its a large animal you'll be in the "zone". If its a rock, I'd click the .5 moa but then what about the other .1 MOA:confused: Somewhere in this scenario the hitters and missers are defined......
     
  9. Michael Eichele

    Michael Eichele Well-Known Member

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    If I understand the situation correctly, the difference is less than 1/4". Yes inch NOT MOA.

    .99 cos is about 5 degrees. This = less than one inch difference for a shot at 750 yards. The bullet will however strike higher even though it is less than an inch. With a 4 MPH head wind at exactly 12'Oclock the bullet will strike lower than normal. Again less than one inch. The main air pattern is likely head on with minimal upwards draft. Any upwards draft in this particular scenario will have extreemly little effect on the bullet. Dial it for 750 with confidence and shoot.

    One of the places I practice is set up exactly as you describe and at the same distance and typically has a head wind. I was shooting some new bullets the first time I went there and they all shot extreemly low. I thought the wind was wreeking havock on them so I went to another location (very flat and on a calm day) and they all hit with the same setting. Turns out it was simply a lower than published BC. So between mathematics and real world testing you would set it for 750 yards and shoot.

    Just my .02
     
  10. AJ Peacock

    AJ Peacock Well-Known Member

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    Meichele figgered it exactly as I did. The updraft is likely only affecting the initial part of flight (less impact), the slight downslope and slight headwind nearly cancel each other out. I'd shoot it dead on 750.

    Nobody has mentioned whether they feel there would be a slight downdraft at the other hillside. If there is, due to a wind eddy from the headwind coming over that hill, I'd hold up .25MOA to compensate for that.

    AJ
     
  11. royinidaho

    royinidaho Writers Guild

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    I'm bettin' that you can take this 2 cents to the bank and make a buck or two.

    When AJ and Meichele agree on something you can surely hang your hat on it!
     
  12. Michael Eichele

    Michael Eichele Well-Known Member

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    Wow!

    Thanks for the vote of confidence!
     
  13. Bravo 4

    Bravo 4 Well-Known Member

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    I got the .25moa less drop from 742 yards compared to the 750 yards. That was from both exbal and JBM. I did account for the down draft on the opposite side, but about as much as I accounted for the up draft on the shooter's side...none.
    My daughter and I drew an illustration of the scenario if anybody wants me to send it to them and they can post it. I'm not smart enough, as the drawing will prove!!!:D
     
  14. AJ Peacock

    AJ Peacock Well-Known Member

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    I put it in my Exbal (I use .8 BC for the 300SMK) and I get -.07 moa for the shot. (to get finer granularity than .25moa, one must change options in Exbal to custom and select a finer granularity than .25moa, I use .01moa).

    I'd recon we all would hit it. When we all agree within 1/4moa, I'd say it's probably close enough.

    Bravo, email me the drawing and I'll post it.

    AJ