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crosswind affects drop

 
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  #15  
Old 08-19-2009, 09:28 AM
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Re: crosswind affects drop

Hmmm... I've read that piece (and ones like it) before, but haven't really noticed the effect per se. Probably due to me being more concerned about the direction change for windage purposes than worrying about whether I'm a little high in the X-ring... if it amounted to more than that, I hope I would have noticed. Then again, if I wasn't really looking for it, who knows if I would have or not?
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  #16  
Old 08-19-2009, 07:28 PM
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Join Date: Mar 2007
Posts: 482
Re: crosswind affects drop

I'll admit up front that I've only skimmed the posts to this thread so I'll apologize if I'm repeating anyone...

There is a vertical component of deflection for spin stabilized projectiles in a crosswind, even a pure crosswind. The deflection is as described above (from 10 O'clock to 4 O'clock). The angle of the slant is proportional to the stability factor of the bullet; the stronger the stability of the bullet, the steeper the slant.

A gyroscopic stability factor of 1.5 (low) will result in about 16 to 17 degrees of 'slant', which corresponds to about .3 MOA of elevation per 1.0 MOA of windage.*

A gyroscopic stability factor of 3.0 (high) will result in about 23 to 24 degrees of 'slant', which corresponds to about .4 MOA of elevation per 1.0 MOA of windage.

The vertical deflection results from a process known as 'aerodynamic jump', which an aerodynamic/gyroscopic 'thing' the bullet does as it aligns it's axis (weathervanes) in a crosswind.

All of the above is the book answer.

One of my favorite quotes is: "In theory there's not difference between theory and practice, but in practice there is".

The book answer can be observed in the real world, and I've seen clear evidence of it myself when shooting 20 shot matches at 1000 yards in different wind conditions. In fact, yesterday I shot in the Palma Individual match at Camp Perry. I had 4 to 5 MOA left windage on at 800 yards and my elevation zero was 3/4 MOA higher than my 'no wind' elevation. At 900 I used 9 to 11 MOA left and my elevation was about 1.25 MOA higher. At 1000 I ran 10 to 11 MOA of wind and my elevation was about 1.5 MOA greater. I'm shooting 185 BT's from a 1:13" twist which results in a pretty low stability factor, so I have a pretty shallow deflection angle. Also the wind was 7-8 O-clock which biased my elevation up some...

There are other real world effects that can act to mask or distort the magnitude of the slant. For example, if there's a component of head or tail wind, that can affect the vertical impact slightly. Also if your sights don't track perfectly horizontal you may unknowingly induce a vertical adjustment to your aim when you add lots of windage. Probably the biggest one is if the terrain that you're shooting over is hilly or rolling; there may actually be vertical wind currents that simply deflect the bullet up or down.

In summary, aerodynamic jump is real, and it does cause a 'slanted' deflection. However, it may not appear the same to everyone in every condition because of other variables involved that are of equal or greater influence.

-Bryan

* Ref: Rifle Accuracy Facts, Harold Vaughn
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Bryan Litz
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And: Accuracy and Precision for Long Range Shooting

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  #17  
Old 08-19-2009, 08:22 PM
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Join Date: Apr 2005
Location: Alaska
Posts: 3,227
Re: crosswind affects drop

Bryan,

So if I've calculated correctly, you just recently observed a vertical crosswind-caused bullet drift ratio ranging from 0.5:4 to 0.67:4 vertical units:horizontal units.

Whereas the initial post described a vertical crosswind-caused drift ratio of 1:4. Which makes the original post in this thread closer to real life experiences than I would have imagined. Interesting and educational. Just another reason to be hopeful, and thankful, for relatively windless conditions at extended ranges. Good thing I don't have to fight substantial winds on a common basis.

Thanks for contributing.
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  #18  
Old 08-19-2009, 09:56 PM
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Location: Northern Id.
Posts: 3,103
Re: crosswind affects drop

This is getting good boys! I guess the only REAL answer is TEST TEST TEST! and judge for yourself......Rich
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