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Thread: Vertical deflection of bullet by wind?

  1. #1

    Vertical deflection of bullet by wind?

    Firstly i'm afraid this will be a lengthy post but hopefully it will be interesting. I have recently tentatively settled on a load for my .20 TAC.

    The load is a 39gn sierra blitz king
    25.1 grains of N133
    CCI 450 primer
    .20 TAC Dakota cases.

    This gives an average velocity of 3775fps

    北北盩his load is safe in my rifle北北北
    I worked up slowly and am getting no pressure signs although i suspect quickload will scream.

    If I concentrate this will produce very tiny groups. The es is too high at the moment but I have only measured one 10 shot string so will see how it looks with a few more rounds tested.

    Anyway, headed out yesterday after a few crows and the only 2 I had a shot at I missed. Now they were 300 yards away and there was some significant wind. This led me to realise I had no experience or skill at judging wind. In the light of this I headed out today to do some practicing.

    The wind was similar today as it was yesterday. So I settled at 200 yards. As an aside, as I approached the field I kept myself behind the hedge in case there was a crow etc in the field. Sure enough there were 2, one at 180 yards and one at 135 yards. I selected the one at 135yards, waited till it moved to a safe place and crowned it with a well placed blitzking. No problem at this range.

    Anyways, I digress.

    I use the "Shooter" app and have found it very good. So I plugged in all the details.

    Now according to a zero height of half an inch, shooter says I should be 0.4 inches low at 200. I am inclined to believe this as when i missed the crows yesterday I saw the strike (about 8 inches down wind! )

    Anyway, the target below shows the actual (5 shot) group. Now I know its not a fantastic group but i haven't done much practice recently and I forgot my rear bag! (excuses over!) The rough drop is 1.4 inches. About an inch more than predicted. Now i know it could be simply that the computer is a bit out but i wouldnt expect this round to drop that much, with this zero height.

    Attachment 33345

    (for some reason the image is rotated 90 degrees left and i cant change it!)

    The main question is could the wind have caused the additional inch of depression. now i only consider this as the field is quite steeply sloped from left to right and as such the wind blows over the top and down, over the line of sight. See picture below. (there is a safe backstop, there is a bank of earth behind the target)

    Click image for larger version. 

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    This topography means that the wind was coming from roughly the 10 to 11 oclock position from the line of the shot but also from the top of the hill to the left and down onto the line of sight. Now because i have little experience of shooting in the wind at extended ranges I was hoping to get some opinion of other people who may have greater experience.

    anyone any thoughts?

    As an aside, the .20 TAC is a joy to shoot and devastating when the driver is on song!

    Thanks very much for listening to the ramblings, there isnt really an issue in real world terms as this would still be acceptable on quarry under these circumstances but i like to know exactly why things happen!

    p.s i dont know what is happening with the picture of the target it has decided it likes it at the bottom!
    Attached Thumbnails Attached Thumbnails photo 1-1.jpg  
    Last edited by hcm1; 10-10-2013 at 21:18.

  2. #2
    ballistic calculators vs real world?

    I had the opposite happen to me! 275yds expecting a drop that didn't appear!

    what Drag function is your calculator set to? G1,2,5,6,7, or 8
    can often change the figures considerably

    assuming you mean 0.5" high at 100yds I have an app that says 0.7" at 200, bit closer to reality but still not exact
    where are you MV figures from?
    some data suggests MV closer to 3600-3650 with that load (stock barrel)

    with that MV a 0.5" high at 100 give 1.05" at 200
    Last edited by bewsher500; 10-10-2013 at 21:38.

  3. #3
    hi mate thanks for the reply, the mv figures are from a chrono although i didnt chrono these specific rounds but the recipie is the same. im using G7 as the drag function as figured the 39gn sbk has a bit of aboat tail. but switching to the G1 does get closer to what appeared to happen. im going to try to repeat the test on a wind free day (not that easy!) and see if there is any difference! the barrel is a 24 inch pac nor 1 in 9. ill post back with any update.

    ​​again, thanks for the reply mate!

  4. #4
    Distinguished Member Ronin's Avatar
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    Oct 2014
    Lancs / Cumbria Border
    The short answer is that a head or tail wind blowing will cause vertical deviation, especially if there are features which cause downdraft over the course of the bullets path.

  5. #5
    Although there is horizontal spread on your target there isn't much vertical. If the wind was causing the vertical changes and, especially as you do not mention adjusting your point of aim to account for the wind, I would have expected the variations in wind to show up as a variation in the drop and as a result significant vertical spread on the target. It seems unlikely to me that you would have experienced enough wind to cause a 1 inch drop but that this wind would be totally constant for the time it took you to fire 5 shots.

    It looks to me like this is where your load shoots at 200 yards.

    In the end ballistic calculators are not exactly and precisely representing the real world as there is a limit to what you can expect in a cheap bit of software with limited resources, the BC quoted for the bullet is probably more to please the marketing department and a chrono is a very cheap bit of electronics that is doing a pretty sophisticated and difficult job without much in the way of regular calibration and so it probably has a potential for significant error. That you can come so close to predicting where the bullet will hit under those circumstances is really very impressive indeed but I suspect that to hope to get within a fraction of an inch in your predictions would be asking just a bit too much, or would involve a lot of luck.
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  6. #6
    It does in Airgun pellets at long range and in a fair blow

    usually clockwise rotation will climb in a right to left wind and fall in a left to right wind.

  7. #7
    I use ballistic calculators/tables as guide lines only. I zero my rifles very carefully in calm conditions and then shoot targets at all ranges in all weather all year round to discover what my rifle and I can do. The results can be very illuminating !

  8. #8
    Thanks for the views everyone, I appreciate the input. Having thought about it again the only way to do it properly is to find a calm day and shoot targets spaced at 20 yard intervals. I guess its true that its quite good that the app gets as close as it did.

    Good point about the lack of vertical spread caorach, i hadnt considered that.

    I'll hope for a still day and will post back with the observations, but i suspect, having thought about it, it will be the same!


  9. #9
    I had to rely on judgement last night, shooting a 20 grain bullet, in a .17 cal, through a crosswind from right to left at 90', gusting around 20mph, Target 150 yards out, deviation was surprisingly minimal ... so practice deffo pays off.
    As mentioned above Apps & calcs are great for guidance, but can throw up some really contradictory results.
    (The Unspeakable In Pursuit Of The Uneatable.) " If I can help, I will help!." Former S.A.C.S. member!

  10. #10
    The biggest problem with wind close to the ground is turbulence and a line of trees can cause very turbulent airflow a good 3 to 400 metres downwind even in pretty light conditions. If the airflow was smooth not so much of an issue, but over a 200 metres a bullet can easily pass through air that is going up, down, left or right from in front and behind and anywhere in between. Try working that out, even if you can measure it all ?

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