Wind Drift formula

Discussion in 'The Basics, Starting Out' started by Cliff Mitchell, Jul 10, 2019.


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  1. Cliff Mitchell

    Cliff Mitchell Member

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    I watch the video today explaining a wind drift formula. It said that the constant of a .308 ranged from 10 to 14. What is the constant for a 6.5 Creedmoor and how do you come up with these numbers.
     
  2. Dog Rocket

    Dog Rocket Well-Known Member

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    For an unknown combination, You have to know the actual drift and then reorder the equation to derive the constant.

    (Wind mph) x (yards/100) / MOA correction = constant.

    ..but there is a better way...

    If using MOA, find the full value wind that drifts your bullet 5 MOA at 1000 yards. You can use JBM Ballistics online, it's free and it works. Or use any other app you might have access to.

    This same wind should give you 0.5 MOA drift per 100 yards.

    For 6.5 Creed, I can tell you that your base full value wind with this method will be about 9mph.

    If you still want to use the old Marine Corps formula you are talking about, your constant will be somewhere around 16 or 17, depending on your rifle and load.
     
    Last edited: Jul 10, 2019
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  3. Cliff Mitchell

    Cliff Mitchell Member

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    Thank you.
     
  4. Greyfox

    Greyfox Well-Known Member

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    In my experience the 6.5CM(260, 6.5x47, etc) seems to fall between the traditional 308 formula of 15 and the typical high BC factor of 20 used for the ballistics produced for cartridges in the 6.5x284 class with velocities in the 2900-3000FPS range. Quite frankly, using an odd number of 16 or 17 somewhat defeats the purpose of a quick mental calculation, and I’d find it more efficient to use a dope sheet, or a MIL scope/formula which is very fast and quite accurate. IMO.
     
  5. D.Stone

    D.Stone Member

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    This is one of the methods I use for my mil based scope.

    Each number represents the miles per hour that it takes to move the bullet half a mil at that distance.

    For instance if I dial for a certain distance, and the nearest number is 4...that means each 4 miles per hour of wind is equal to half a mil of hold.

    The same idea is easily applicable to minutes of angle. Just figure out the elevation settings to correlate to x mph = 2 MOA.
     

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  6. Dog Rocket

    Dog Rocket Well-Known Member

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    Interesting approach, I don't think I've seen that one. Thanks for posting it.
     
  7. Greyfox

    Greyfox Well-Known Member

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    The situation where I find wind corrections most intense is in PRS competition. The method I use which is fast and easy with my MIL based scopes is to calculate the full value wind speed that produces a 1MIL wind correction(w/o spindrift) at 1000 yards. Correction for other ranges are simply in tenths of a MIL. For example for my 6.5x47 the wind speed for a 1MIL correction at 1000 yards is 6MPH. At 400 yards the 6MPH wind correction would be .4MILS. at 900 yards it would be .9MILS, etc, etc. A correction for angular wind direction can be easily accounted for. If that 400 yard dope was for a 12 MPH wind at half value, multiply the .4MIL correction by 2(.8MIL)for the 12MPH wind, then divide by 2 for the half value(.4MIL)....in this case the wind correction would still be .4MILS.
    The wind speed for a 1MIL calibration at 1000 yards for my 6.5x284 with its higher ballistic performance is 10MPH.
     
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  8. Dog Rocket

    Dog Rocket Well-Known Member

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    This is my preferred method also for MILS.

    The closest you can get to that level of ease in MOA, is to use the exact same methodology, but substitute 5 MOA at 1k yards in place of 1 MIL. And use that wind as your base wind.

    This gives 1/2 MOA per hundred, so your wind call is basically the first number of your yardage divided in half.

    600 yd = 3 moa
    700 yd = 3.5 moa
    800 yd = 4 moa
    1000 yd = 5 moa
     
  9. D.Stone

    D.Stone Member

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  10. Cliff Mitchell

    Cliff Mitchell Member

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    Thanks that makes it kind of simple
     
  11. Cliff Mitchell

    Cliff Mitchell Member

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    I want to thank all you gentlemen for your help now I'm just going to have to figure out which one works best for me. I picked up the rifle yesterday so I'm one step closer now I need to just pick out my glass which I think I know what I'm going to go with
     
  12. ladebenken

    ladebenken Member

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    This is the formula i use for competition shooting with the 6,5x55 swede, loaded with 9,0gram bullets:

    Number of hundred meters= range in meters, divided by 100. ex. 500 meters = 5
    Full value wind= in m/s ,, meters per second.


    Number of hundred meters x full value wind / 2 = click (0,1 MIL)

    Ex:
    Range is 500 meters
    Full value wind is 5 meters per second

    ;
    5 x 5 /2 =12 click or 1,2 mil.

    This simple formula is usually within a click from the ballistic calculator, and that is good enough.