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

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by ICANHITHIMMAN, Apr 23, 2009.

  1. ICANHITHIMMAN

    ICANHITHIMMAN Well-Known Member

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    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?
     
  2. edge

    edge Well-Known Member

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    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.
     

  3. Buffalobob

    Buffalobob Writers Guild

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    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.
     
  4. Michael Eichele

    Michael Eichele Well-Known Member

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    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.
     
  5. Long Time Long Ranger

    Long Time Long Ranger Well-Known Member

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    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.
     
  6. ICANHITHIMMAN

    ICANHITHIMMAN Well-Known Member

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    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: Apr 24, 2009
  7. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    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
     
  8. JLR

    JLR Well-Known Member

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    We've shot a number of the JLKs out of a 6.5x47 at around 2950fps and have found the .620 BC to actually be pretty close. My brother, TMR, could probably shed more light on this as he's shot them a lot more than I have. I was just getting into working up a load for them when I sold my 6.5 a few weeks ago. I will say that they are a really consistent bullet and shot incredible out of everything we've put them in so far.
     
  9. Ridge Runner

    Ridge Runner Well-Known Member

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    of all the bullets I have shot, only one has been right on as far as my actual BC compared to what the maker claimed.
    Now, As long as you have everything else right in your program, (MV, scope height, ect) and you have your current conditions right, enter everything into your program. I zero at 300 yards, then shoot at 400, 500, and 600 yards and measure my actual drops. I then check my chart almost always I'm dropping less than the program says I will, I then start changing the BC in my program till my chart matches what I'm actualy shooting.
    Then I adjust for 600 yards, confirm zero and shoot at 700, and 800 and confirm my drops, may do it again and make sure and possibly tweak the BC a lil more, then I'm done, thats the BC I enter for that bullet in my conditions.
    Now here are my results
    In my 7mm AM I shoot a 160 gr accubond at 3575 fps, nosler claims a BC of .531, I've been running this bullet out as far as 1350 yards using a BC of .62 and POI is dead on POA near as I can tell.
    In my 6.5 Gibbs I shoot a 140 berger VLD at 3240 fps, published BC of .64, POI matches POA out to 1060 yards using a BC of .7
    Have no idea if this is right but it works very well for me.
    RR
     
  10. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    Ridge Runner,

    My friend, your going to get in trouble posting BCs like that!!!:D

    I can already hear the keys rattling around the world getting ready to hammer you for saying a 160 Accubond has a BC of over .6!!!!!

    All kidding aside, the given BC value is simply a starting point with any bullet. Use that to get started and then you will have to tweak alot of things to get the computer generated models to match up to actual bullet trajectory.

    Also, as mentioned, BC changes as velocity changes so its not the same at close, mid or long range. In my testing, its common to see BC be lower from 0 to 200, increase a bit from 200 yards out to until the bullet reaches around 1200 fps and then drop off dramatically as the bullet drops under 1100 fps.

    Its all an average. Find a number that puts you on target over the trajectory of your combo and your there, the actual number you use is meaningless as far as describing the actual BC of a given bullet, its just a value you have to use to predict where your combos bullet will be at any range on its trajectory.
     
  11. ICANHITHIMMAN

    ICANHITHIMMAN Well-Known Member

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    Man I have so much to learn about BC thanks guys.
     
  12. Ridge Runner

    Ridge Runner Well-Known Member

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    Yeah kirby I hear ya, hard for me to believe also but its what works, and it works well. I may start a post asking the rest of the guys what BC they get from the 160 accubond. The numbers seem unreal but I like hitting what I shoot at.
    RR
     
  13. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    Amen, the idea is to hit what your aiming at!
     
  14. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Ya know... I think that would be a great idea. I wonder if we could get a sticky thread going for folks to list what BC's they are getting for different bullets?