Wind and Spindrift

Discussion in 'Long Range Hunting & Shooting' started by Litehiker, May 13, 2019.


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  1. Canhunter35

    Canhunter35 Well-Known Member

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    Actually, I’ve found the faster twist barrels enact more spin drift on the bullets. My Creedmoor requires more spin drift compensation than a 1:11 308
     
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  2. Dog Rocket

    Dog Rocket Well-Known Member

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    As I understand it, there is great interplay between Sg and CD in determining that. Longer bullets with higher BC's require greater twist, which (all else remaining equal) increases the angular momentum, which increases potential for drift.

    However, if greater stability (2.0 Sg or higher) can be achieved without damage to the structural integrity of the bullet, then the long cycle coning motion around the mean axis of trajectory can be decreased. This increases the BC further, which shortens flight time, which decreases the time the bullet has for drift. Thus, it offsets the increased angular moment to differing degrees depending on the combination.

    I was using 1% for my fast twist 1/8.5 338AX out to 2,400 yards, and 2% for my 308 Win out to 1200 yards.

    But what I am coming around to presently, is just using 1.5% for all and just realize that the error will be so much smaller than the other variables I have to worry about, that it isn't worth fretting over.
     
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  3. Dog Rocket

    Dog Rocket Well-Known Member

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    That may very well be the case with your combination. It isn't something that can be said for every case. The take away is that whatever it is for your combo, that number is discoverable and calculable in your head very quickly due to it being some invariable percentage of drop.

    Whether it is 1% or 3% is irrelevant as long as it can be figured quickly and accurately.
     
  4. Doug Herold

    Doug Herold Well-Known Member

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    Something you may want to consider:
    Set your zero .25 moa (1 click) to the left of center @ 100 yds. This puts me well into the kill from 100 to 800 yds. (POI will drift back an inch or so to the right at 800). But I don’t have to complicate the thought process with the wind drift call out to that distance. At 1000 yds, my POI is going to be @ 3” to the right so I have to “hold” for that. Most misses are because the shooter doesn’t hold for enuf wind — not because he held too much. More important is the correct wind call. To add the wind drift into the calculation just complicates the process and costs valuable seconds... just IMO.
     
  5. Greyfox

    Greyfox Well-Known Member

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    I used to apply the various calculation factors for setting wind and spin drift and had decent success. Of late, ballistic calculators like Applied Ballistic and Hornady 4DOF do a masterful job of delivering precise ballistic holds, including spin drift when set up precisely. I will take into account all environmental factors(including fine adjustments for wind speed and direction) when setting/checking 100 or 200 yard zero’s. Once velocity/BC is verified at long range....all that remains is shooting straight, and an accurate wind determination.....the hard part!!!
     
  6. Pro2A

    Pro2A Well-Known Member

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    Greyfox...that ole wind certainly separates the shooters from the trigger pullers!!! Ever notice how no one considers rifle cant? Brian Litz, Applied Ballistics, writes that research indicates few individuals can perceive closer than 3degrees cant.....which equates to about 42" at 1000yds.....plus any field optical illusions of vertical reference......and, of course, haste and buck fever. I'm betting that more misses are due to cant than anyone realizes. Even typically available vial levels (Vortex, NF, Accuracy 1st,...) are only about 3 degrees accurate. Electronic levels employ inclinometers that can be calibrated to 1deg or less....ie Sig Tango 6 in-scope LevelPlex or the Send-It unit (see Brownells). We focus on the obvious details and forget the human limitations.
     
  7. Greyfox

    Greyfox Well-Known Member

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    Yes, I have always been a stickler on rifle cant and meticulous about my rifle/scope set-ups. I use a bubble level which I’m able to keep cant under 1 degree. I really like the anti-cant design on my most recent build using an MPA Chassis.
    227E5D39-9CAF-4979-B1B5-58F7658197A0.jpeg
     
  8. Dog Rocket

    Dog Rocket Well-Known Member

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    If you are missing low as well as right or left, then it could be cant. If you are missing off the edge at 3 or 9 o'clock, then cant isn't your issue.

    Bryan Litz either pulled the 3 degrees number out of his a**, or he wasn't measuring experienced shooters to begin with. 3 degrees is huge. Optical illusions can trick anyone, but those aren't the norm.

    Removing variables isn't a bad thing. Why worry about parallax when a shooter can only call wind to the nearest 5 mph? Why worry about follow through? Why have a 2 lb trigger? Why strive for a consistent cheek weld? Why worry about narural point of aim?

    None of that stuff amounts to huge misses by themselves. Together they might more than double your dispersion. Spin drift is easy to get rid of, a simple twist of a dial. The other stuff is hard. No instructor I've had has ever advised me to ignore the easy stuff until I had mastered the difficult.
     
  9. Wedgy

    Wedgy Well-Known Member

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    One of the best posts.... dial 0.5 left and worry about the other stuff
     
  10. Pro2A

    Pro2A Well-Known Member

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    Greyfox, what level are you using? Don't recognize the design. Is it a MPA proprietary item? Is that a ceramic bead? Is it a viscous or thin fluid....slow or quick response? Pic appears wide between lines and bead. Have you confirmed it with a known precision machinist level or digital inclinometer...ie Starrett ? Interested in what others find in level accuracy. We assume many things are what we want them to be. We pick up a level and assume it is accurate. Most shooters assume their scope tracks accurately and repeatability. Interested in your feedback. Thanks.
     
  11. Litehiker

    Litehiker Well-Known Member

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    Keerect, canting is the bane of long distance shooting. I think it is just as often the cause for misses as is missing the wind call - and it's likely often mistaken for a "wind shift". ;o)

    Both my 6.5 CM Ruger Precision Rifle and my 6.5 CM Browning X-Bolt Pro have bubble levels. The RPR hangs off the scope to the left and the X-Bolt Pro is integral to the Talley top ring (nice).

    Even now, after a few years of having a level on my RPR, I STILL have to remind myself to check the level.
    AN INTEGRAL LIGHTED RETICLE LEVEL, as in a SIG Tango 6, is the best way to go B/C you never forget to use it. Thank you SIG (and Leupold).

    Eric B.
     
  12. Greyfox

    Greyfox Well-Known Member

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    The level is included on the most recent MPA Chassis I purchased, replacing the same type level using an air bubble. The ball appears to be ceramic, and I’m unsure about the type of the fluid...but it doesn’t freeze. The location, IMO, is far superior to scope tube mounted levels, and the black ceramic is highly visible. It is very closely centered/matched to the leveled action in the chassis. I simplely plumbed my scope reticle with the ceramic ball centered. It has proven to be consistent, and very fast/easy to center without coming off my cheek weld. Much like a peep sight, the placement of the indexing lines help center the ball naturally to the eye. It’s a nice design that is very precise and compliments the fast action and varied targets encountered in PRS competition.