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Drag Model

 
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  #1  
Old 02-25-2009, 04:56 PM
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Location: Jackson Ms
Posts: 316
Drag Model

How do you know when to use which model? Does it matter? I know it does but kinda frustrated. G7 for berger vld right?

Somebody point me in the right direction before I pull what is left of my hair out.
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  #2  
Old 02-25-2009, 10:08 PM
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Location: Patagonia Mountains, Arizona
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Re: Drag Model

Quote:
Originally Posted by Limbic View Post
How do you know when to use which model? Does it matter? I know it does but kinda frustrated. G7 for berger vld right?

Somebody point me in the right direction before I pull what is left of my hair out.
The ballistic coefficients published by all bullet manufactures are based on the G1 model unless otherwise stated. (and it's rarely stated otherwise). Most bullet manufacturers don't even state what velocity the stated G1 BC is correct for, but the coefficient are generally close in the 2500 to 3500 fps range and usable from about 1500 to 4000. They're terrible in the transonic range. Sierra is unique that they usually publish 3 or 4 BCs for ranges of velocities. You'll find flat base spitzers fit the G1 curves fiarly well.

Why does everyone use G1? Pure marketing. The G1 numbers are bigger because the test projectile was higher drag than any other model. Wouldn't you want to buy a bullet with a 0.65 G1 BC than a 0.31 G7 BC? Just remember that while BCs do predict wind deflection, drop, and energy loss. They do not predict still air accuracy. Most accuracy records are set with relatively low BC bullets.

Dont agaonize over BCs Using G7 or G5 won't give you a significant improvemnt in results and you'll have to develop the measurements yourself if you wnat to use them There are just about no published values other than G1. Other G functions may be useful when trying to precisely "curve fit" you measured drops at different ranges then use the progam to fit other points in between. That may be useful if you shoot one bullet a lot, but you'll find it still won't fit perfectly.

If you want to understand what BCs mean and where BCs came from read "Modern Exterior Ballistics" by Robert L. McCoy of BRL. (Aberdeen Proving Grounds). THe book also discusses the limitations of ballistic computations. BCs arent the only limiting factor. . Even if drag tables were perfect there is a whole layer of calculaton limitiatins which aren't even addressed by conventional ballistic programs which would still limit accuracy. Those are the result of bullet yaw and precession. The math is available to do the more advanced calculations but there is no way to predict the input conditions accuratly enough for typical shooting to make those methods useful. It's also why you'll never see a spin stabilized rifle which shoots perfeclty . It's amazing some do as well as they do.
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  #3  
Old 02-25-2009, 11:30 PM
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Join Date: Jun 2008
Location: Jackson Ms
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Re: Drag Model

That is one hell of a response and I thank u. The ballistic program for my iPhone has the different models and a brief explanation listed to the side. It specifically says G7 for VLD.
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  #4  
Old 02-26-2009, 08:38 AM
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Posts: 508
Re: Drag Model

Limbic,
I have G7 BC's for Bergers and some other brands as well that are based on test data and are accurate within +/- 1%. If you have a particular Berger bullet in mind, I can provide you with the G7 BC. If it's a non-Berger bullet, I might have data for it.
Lou is correct about flat based bullets being a better match to G1, so if you're looking at flat based bullets don't worry about G7.
Most long range bullets with boat-tails (secant or tangent ogives) match G7 better than G1.
Lou is also right about conventional ballistics programs not accounting for all of the subtleties of a spinning bullets flight. However, if your bullet has adequate stability you can predict a very accurate trajectory out to 1000 yards by entering accurate inputs into the program. Most programs won't account for spin drift, coreolis acceleration, or other effects that are 'second order'. You'll get a very good trajectory from a conventional program that will probably be within the accuracy of your equipment.
Considering the second order effects can help to improve your chances of a first shot hit, but as Lou stated, the inputs required and the cost of a program that's capable of doing the math is prohibative.
Here's a link to a couple articles that talk about some of the effects not considered by typical ballistics programs, and the affect they can have on your max effective range:
Homepage of Bryan Litz - A Bravenet.com Hosted Site
Click on 'Max Effective Range' on the left.

-Bryan
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And: Accuracy and Precision for Long Range Shooting

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  #5  
Old 02-26-2009, 12:33 PM
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Join Date: Feb 2008
Location: Wampum, PA
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Re: Drag Model

Bryan,

That's article is a real eye opener.
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  #6  
Old 02-26-2009, 11:17 PM
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Join Date: May 2007
Location: Alabama
Posts: 157
Re: Drag Model

Bryan, can you give me the G7 BC numbers for the 6.5mm 130 and 140 VLD's?

Thanks,
David
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  #7  
Old 02-27-2009, 06:59 AM
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Join Date: Mar 2007
Posts: 508
Re: Drag Model

David,
They are:

G7 BC for the 130VLD = 0.282
G7 BC for the 140VLD = 0.304

-Bryan
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Ballistician

Author of: Applied Ballistics for Long Range Shooting
And: Accuracy and Precision for Long Range Shooting

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