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BC Decay Rates

 
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  #1  
Old 12-30-2009, 11:34 AM
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Location: Afognak Island, Alaska
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BC Decay Rates

I was going to email this question to Michael Eichele since I'm using the ballistics program he gave me, but I thought I'd throw this out to anyone.
How do you come up with the ballistic decay of a particular bullet? Why is it included in some programs? I'm assuming BC is measured right out if the barrel and two different bullets having the same BC might have a different BC decay rates which would result in different trajectories downrange. Am I on the right path here? Are decay rates of different bullets published anywhere?
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Old 12-30-2009, 11:43 AM
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Re: BC Decay Rates

LR,

The fact that a BC changes (decays) as it flies downrange is an unfortunate consequence of the BC being referenced to a non-representative standard (G1). If you reference the BC to a more appropriate standard (like G7 for long range bullets) the BC doesn't change (significantly) with velocity.

In order to use a G7 BC, your program has to be able to accept it (some programs only work with G1). I don't know if Michael's program can use G7 or not but there are plenty that can including this one:
http://www.appliedballisticsllc.com/...Ballistics.exe
*email me if you have trouble with the download (bryan.litz@bergerbullets.com)

Here's an article that explains more about G1 and G7 BC's.
Berger Bulletin Blog Archive A Better Ballistic Coefficient

The truth is that we shouldn't have to deal with BC decay; there's a better way.

Berger provides G7 BC's for our bullets on the webpage. I've also measured and published G7 BC's for ~175 LR bullets of all calibers and brands in my book.

To answer your question directly, there is no way to reliably predict the G1 BC decay rate.

Take care,
-Bryan
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Bryan Litz
Ballistician

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


Last edited by BryanLitz; 12-30-2009 at 05:03 PM.
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Old 12-30-2009, 12:07 PM
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Re: BC Decay Rates

Thanks Brian. I checked Michael's program and the field can be changed to another decay rate. Right now it shows a G1 decay rate of 0.290.
I've noticed that you list both G1 and G7 BCs for your bullets. If I wanted to enter your G7 BCs for my 95gr 6mm VLD load, I would have to change the decay rate also, right? Would I just enter zero in that field?
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Old 12-30-2009, 12:51 PM
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Re: BC Decay Rates

I'm not familiar enough with Michaels program to answer that, but I'm pretty sure you don't want to enter zero.

I know that if you use a program that specifies G1 and G7 it will reference the BC to one of those standards.

-Bryan
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And: Accuracy and Precision for Long Range Shooting

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