9 o'clock vs. 3 o'clock wind drifts different with same wind velocity???

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by shortpants, Feb 10, 2012.

  1. shortpants

    shortpants Well-Known Member

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    I'm getting 52" of drift with a 9 o'clock vs. 33" of drift with a 3 o'clock wind using bullet flight on my iPhone. I'm assuming these numbers are different due to spin drift with a right hand twist but didn't think the difference would be that great. Am I doing something wrong? This was a 5mph wind @ 1000yds.
     
  2. pyroducksx3

    pyroducksx3 Well-Known Member

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    Not sure why you're getting that. I get the same going either way on mine, only difference is -2moa or 2moa depending in wind position. I'm using bullet flight fte on my ipod
     

  3. Broz

    Broz Well-Known Member

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    Something is wrong. Probably an input parimeter or a lead installed like a moving target app. Spin drift at 1000 should be more like 5 to 7 inches. Go over all your settingand see if one needs zeroed out.

    Jeff
     
  4. Topshot

    Topshot Well-Known Member

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    52-33=19
    divide by 2 = 9.5"

    Seems a bit much for Spin drift even with Coriolis thrown in as well. But I just punched the numbers into "Shooter" for a .308 with 155gn bullet and they are similar for a Northern Hemisphere location when SD and CE are taken into account.
     
  5. shortpants

    shortpants Well-Known Member

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    Both spin drift and coriolis are being accounted for. Also this was 180gr. Fusion ammo B.C.=.478 going 2950
     
  6. shortpants

    shortpants Well-Known Member

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    Another interesting fact is that 52" of drift @ 5mph becomes 94" when 10mph is plugged in. I would think it would be 104". Ive gone through this app and can't find what could be causing these readings. If I take spin drift off and coriolis off then the drift is the same whether it be a 3 o'clock or a 9 o'clock. It does make sense that with a no wind hold the bullet would drift right (and low) with a right hand twist so I could understand a different hold when dealing with a 90* or a 270* wind. I just have a hard time believing it would be as great as what it is telling me.
     
  7. SBruce

    SBruce Well-Known Member

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    That does seem excessive. The numbers I've seen generated from Exbal suggest only a 3" effect at 1000 yds due to Coriolis alone. Spin Drift (as near as I've been able to gather) could perhaps be 3 times that amount in some cases.?.........Still, we're only talking about maybe 12" difference at 1000 yds, not 19".

    Every app that I've personally looked at also suggests that a 10 mph wind doubles the effect of a 5 mph wind. Hmmmn.....??
     
  8. Topshot

    Topshot Well-Known Member

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    Remember you have to half the drift difference to compare it against SD and CE.
    SD is added in one direction and subtracted from the other as it is always to the right with a RH twist barrel.
    So the 19" difference is actually 9.5" total drift due to SD and CE.
     
  9. SBruce

    SBruce Well-Known Member

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    AhHaaaa, that makes sense. Good point. Thanks for that insight!

    If I'm understanding correctly, in theory we'd have 9-10" right at 1000 under no wind for SD and CE, dead on with a 3:00 wind and 20" right with a 9:00 wind, assuming the wind alone accounts for 10" drift at 1000.

    That seems to also explain why when we were shooting at 1000 yds (with a 4:00 wind that increased speed as the day went on) that our shots continued to climb higher than expected (as the wind increased) and the wind didn't blow them off as much as expected.
     
  10. Topshot

    Topshot Well-Known Member

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    Yes, the first part is correct for that scenario.

    Not sure about the second part with the shots "climbing higher"? That could be due to a vertical component of the wind?
     
  11. shortpants

    shortpants Well-Known Member

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    Thanks for clarifying that Top Shot that makes since. I ran some different numbers to help make sense of it.
    A no wind hold puts me 9.2" right
    A 1mph 3 o'clock wind puts me .7" right
    A 1mph 9 o'clock wind puts me 17.8 right

    A 1 mph wind has an effect of roughly 8.5" of drift
    A 3 o'clock wind you subtract 8.5 from 9.2 since the wind is pushing the bullet back against the spin drift almost canceling eachother out which gives you .7

    For a 9 o'clock wind you have to add 9.2 and 8.5 since they are working together which gives you 17.7 (close enough). Not sure why the .1 difference but understand the answer to my question now.

    I love this website!!!
     
  12. SBruce

    SBruce Well-Known Member

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    It sure could be, I haven't yet totally proved to myself that horizontal wind causes vertical drift, but it seems to be a more recognized phenomenon here lately. I even started this thread a while back to try and get some answers.
    http://www.longrangehunting.com/for...-changes-our-vertical-impact-fact-myth-51306/


    In the video How to Dope the Wind Beyond Belief, one guy talks about how the spinning bullet drops less with a right wind, and drops more with a left wind. Just as spin drift "rolls" the bullet right (RH twist), it supposedly also makes it drop less or more depending on the wind direction. He stated that for every 3 or 4 minutes of drift, there would be about 1 minute of elevation change.??

    When I get my LRR all load picked and dialed in at 1000, I intend on testing this theory for good. I can shoot 1000 yds east and then turn right around and shoot 1000yds west in no wind conditions. Then on another day, when there is a slight wind from the north or south, I will shoot the same test at the same distances and try to see just how much the wind changes my vertical........problem is, on different days there will be a different pressure or temperature to deal with too. If Coriolis affects the vertical (like the program says) that's 6 inches vertical that would have to be accounted for. Might take a long time and alot of ammo before I can totally prove this theory right or wrong. Expensive too at $5 a crack and a new barrel after $1500 rounds.:rolleyes:

    In addition to the chart in the above link, here's another one that has been posted here on LRH lately. Even if it's never proved, it's interesting to discuss.
    None of my programs address this phenomenon at all.
     

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  13. Topshot

    Topshot Well-Known Member

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    Not sure if I am reading your chart properly but there seems to be a big change in POI when the wind is going from indication points #6 and #7. Also the same large change in POI when the wind is going from points #12 and #1.
     
  14. SBruce

    SBruce Well-Known Member

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    Yes there is, hadn't noticed that before.?? That's weird.

    This chart isn't quite so drastic. I didn't make either one of these, just passing them on for sake of conversation. I don't yet shoot 1000 yds and beyond often enough to have my own interpretation of what's really happening out there.
     

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