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BC Decay Rates

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  #1  
Unread 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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  •   #2  
    Unread 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

    Last edited by BryanLitz; 12-30-2009 at 05:03 PM.
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      #3  
    Unread 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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      #4  
    Unread 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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