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How do I get the BC for a bullet whos maker dosent even know????

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  #1  
Unread 04-23-2009, 07:34 PM
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How do I get the BC for a bullet whos maker dosent even know????

I am looking at the 6.5mm 140g JLK VLD BC .630 and the 130g JLK VLD BC.620?? as the website reads.

I emailed them and they said they dont know the exact BC of the 130g so how do you find out. Is there a mathmatical formula?
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  •   #2  
    Unread 04-23-2009, 08:20 PM
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    Re: How do I get the BC for a bullet whos maker dosent even know????

    I will guess around 0.585

    Assuming the same shape, the BC should be proportional to the mass.

    0.630 / 140 * 130 = 0.585

    edge.
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    Unread 04-23-2009, 08:40 PM
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    Re: How do I get the BC for a bullet whos maker dosent even know????

    This does a reasonable job.

    Precision Ballistic Coefficient Estimator

    But if you already know the approximate BC from the JLK website then you just go and shoot your drops out to 1K or so and see how things work out.
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    Unread 04-23-2009, 09:44 PM
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    Re: How do I get the BC for a bullet whos maker dosent even know????

    On the Lilja website you can download an excel file for twist and BC.

    You can enter many key measurments of a given bullet and it will give you an approximate BC. Close enough to get you on paper.

    As a side note, even bullets that have a published BC attached to them need to be tested. It will vary from rifle to rifle and from velocity to velocity. Most if the time published or calculated will get you on paper so you can fine tune the number yourself.
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    Unread 04-24-2009, 09:15 AM
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    Re: How do I get the BC for a bullet whos maker dosent even know????

    I do two chrono's. One at say 15' in front of the muzzle and the other downrange at a known distance. Once you have the velocity at two known distances you have your true BC. The BC will change slightly as the bullet loses velocity downrange. In other words you will get a slightly different BC from 0-500 yards than you will at say 500-1000 yards. If I want to know the BC at a certain velocity range I load it down to that velocity instead of shooting over a chrono at a 1000 yards. Hope this little soundbite helps a little.
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    Unread 04-24-2009, 09:34 AM
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    Re: How do I get the BC for a bullet whos maker dosent even know????

    The 130g and 140g VLD are dimensionaly the same the only diffrence in the bullet is the 10g of weight. I tryed to use the estimator
    with this info but I didnt have any luck.


    Last edited by ICANHITHIMMAN; 04-24-2009 at 12:31 PM.
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      #7  
    Unread 04-24-2009, 09:47 AM
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    Re: How do I get the BC for a bullet whos maker dosent even know????

    I haven't tested these yet, but based on my dimensional measurements of the 140 JLK (not the dimensions given on the website), I would estimate a BC of 0.617 for the 140, and 0.573 for the 130 (if the 130 has the same ogive and boat tail as the 140). These BC's are averaged from 3000 fps to 1500 fps, and are likely within +/- 5% of the actual values.

    I'm interested to learn how these estimates compare to other estimates or test results.

    -Bryan
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