Ballistics Question

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by jeffro, Nov 7, 2009.

  1. jeffro

    jeffro Well-Known Member

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    Im dialing in a few guns for long range shooting. I have a CED crony. Im taking my vel, and putting it into JBM. Im using thier BC for the bullets. All the temps, bar, Hum, and alt are correct. When I get out too 700 to 800 yrds and beyond, the charts are off by a little bit. .5 MOA at the most. Im wondering what others do to tune thier charts. Do you adjust your bullet speed or the BC of the bullet.

    Thanks.
     
  2. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    Jeffro,
    The most common reason for 'observed' trajectories being different than 'calculated' trajectories (assuming of course that the inputs to the calculation are truly accurate) is sight adjustments.

    Most shooters take for granted that when they dial 30 MOA on their turret that the crosshairs actually move 30.00 MOA. However, in many cases, the crosshairs don't move exactly as advertised. You're very likely to get +/- 1/2 MOA or more error when you make large sight adjustments. This results in the bullet striking somewhere other than you're point of aim, but it doesn't necessarily mean the bullet's trajectory was off from the calculation, maybe your point of aim is off from where you think it should be.

    One easy way to test for this is to set up a 'tall target' at 100 yards. Put an aimpoint at the bottom of the target and shoot a group at your 100 yard zero. Then move your scope up 20 or 30 MOA and shoot another group. Measure the distance between the two groups and see if the point of aim actually moved exactly according to your adjustment. If not, you can calculate a 'calibration factor' to apply to your sight adjustments.

    The 'tall target test' is also a good way to insure if your scopes vertical adjustment is tracking perfectly vertical. Draw a line on the target using a plumb line or level, then see if your vertical adjustment produces two groups that are parallel to the line.

    Some scopes are right on the money with their adjustments, some can be off by more than 10% (meaning a 20 MOA adjustment would actually move 18 or 22 MOA). I believe this is the most likely cause of your perceived error.

    Another thought, what bullets and BC's are you using from JBM? Some of the BC's in that library are more accurate than others.

    -Bryan
     

  3. jeffro

    jeffro Well-Known Member

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    The bullets that Im using are.

    Horn- .308 168 Amax
    .308 208 Amax
    .284 162 Amax

    Nosler .308 168 CustomCom Match
     
  4. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    jeffro,
    Each of the bullets you mentioned is available in the JBM library with a 'Litz' measured BC. When you choose that bullet, the measured G7 BC for that bullet is loaded which, next to doplar radar data, is the most accurate and best fitting BC for long range bullets.

    My measured BC's are typically good enough to predict trajectories with less than .5 MOA of error out to 700/800 yards. I suspect the ~.5 MOA error you're seeing is mostly due to imperfections in scope adjustment, or uncertainties in some of the other inputs.

    How many shots are you shooting to determine the group center in relation to your aim point?

    To put it in perspective, .5 MOA error in predicted drop isn't very bad at 700/800 yards.

    It would only take a 2.5% calibration error in your scope's reticle movement to create the .5 MOA error at 800 yards. If you dial 20 MOA and the scope moves 19.5 or 20.5 MOA, you should be able to detect that with the 'tall target' test.

    To answer your original question: how to fix it?...
    There's no easy way in JBM. Let's say you do the tall target test and find that your reticle moves .256 MOA per click instead of .250 MOA per click. You would have to manually convert the drop table from inches to clicks based on the correction factor that's specific to your scope. This can be done in a spreadsheet.

    Just to cover this base; are you using the correct click value in JBM? If you selected 1/4" clicks vs 1/4 MOA clicks (or vice versa) that would be more than enough to cause your perceived .5 MOA error at 800 yards.

    -Bryan
     
  5. Ridge Runner

    Ridge Runner Well-Known Member

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    If your good to 700 yards its probably a BC issue, change the BC on JBM until your charts match your trajectory, it works for me.
    RR
     
  6. Eaglet

    Eaglet Well-Known Member

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    Jeffro,

    Do your self a favor, follow everything bsl135 is advising and if that doesn't

    work then follow what RR is saying. :)

    Regards,
     
  7. jeffro

    jeffro Well-Known Member

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    Thanks for all your help. I will look into all of this.