trajectory calculator help

Discussion in 'Long Range Hunting & Shooting' started by Round2it, Sep 15, 2012.

  1. Round2it

    Round2it Member

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    Help!
    I'm having trouble getting that JBM simplified trajectory calculator to work for me. I'm attempting to duplicate my real world results with my 223. At 290 yards I dial 2.5 MOA from my 100 yard zero and am right on. The calculator says it should be 4 MOA! I just shot this again today for verification.
    Here's what i'm shooting.
    Bushy Predator 20", 2945fps, 60gr. V-Max over 25 gr. of Varget.
    I'm shooting from a bench with my targets at my same elevation. My scope sits at 1.75" above the bor. 75 to 85° F at 550 ft. with about 65% humidity. I'm useing a Leupold 4-12 Mark AR with 1/2 moa clicks. I'm using the bullet library and imputing what I have listed here.
    What am I doing wrong?
    Thank for your hep.
     
  2. JackinSD

    JackinSD Well-Known Member

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    What I see is that you will need to validate your real world trajectory and at least one more distance. Once you have done that, or even before, you will either have to change your velocity, or BC (usually velocity) in your ballistics program to math your real world trajectory.

    In short word, validate.
     

  3. Eaglet

    Eaglet Well-Known Member

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    4-4.2 MOA is about right... with 1/2 moa clicks you'd have to round down to 4.0 MOA and go 8 clicks up...

    Now, make sure the center of 3 shotS group matches with the center of the target at
    100 yards; then go 8 clicks up and still shoot at 100 yards, if the center of the group
    is not between 4 and 4.6" high then it would give you a pretty good idea about how
    the scope is tracking. If it prints where expected then shoot it at 300 yards and it should
    be very close.

    Do pay attention and take in account how small of a group the shooter/rifle combo are capable of.
     
  4. Round2it

    Round2it Member

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    Eaglet,
    I have done that twice now. I'm only having to move up 5 clicks and at 100 yards it prints 2.4"high.
    Correction: my scope height is 2.6" Not 1.75".
    For the other yardage test I would think that farther is better than close, or would 200 work just as well as long as I'm very accurate with the measurements?
    Just in case it matters, my 10 shot ES is 34 and SD is 11.
    I always shoot a minimum of 5 shot groups.
     
  5. JackinSD

    JackinSD Well-Known Member

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    The further out you can do the validation the better. I try to do mine every 100 yards out to the distance that I feel comfortable shooting. The more accurate data, the more accurate your ballistic calculators will be.
     
  6. Round2it

    Round2it Member

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    I will get that done next week see what results I get.
    Thanks
     
  7. Eaglet

    Eaglet Well-Known Member

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    Alright, I used JBM and punched in all info. 29.9 for pressure, 2.6" for scope height
    and to 290 yards, it calculates 3.4 MOA of adjustment needed. That would put us at
    7 clicks. U're already using 5 clicks so for that distance one full MOA is too much of a difference...

    I think that factory ammo in that bullet weight has a muzzle velocity of about 3150 ft/sec
    ...thinking aloud here.. If you were to have a muzzle velocity of 3250 ft/sec then you
    would only need 5 clicks up to be dead on @ 290 yards... Can that be possible?
     
  8. Round2it

    Round2it Member

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    I checked my chrono with some Eley 17 mach 2 loads that ranged from 2120 - 2258. That's prettyclose to the factory specs. .
     
  9. JackinSD

    JackinSD Well-Known Member

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    If your velocity of 2945 was correct and you are using a 60gr v-max, you would have to have more than doubled it's BC to get that trajectory (.600 instead of it's published .265). I will assume the 2.6" sight height is correct.

    I doubt that the published BC is off by much.

    That leaves only a these of other possibilities:
    a. Velocity
    b. Sight in distance.

    You are sure of your sight in distance of 100. That leaves velocity as the only other possible to create that kind of difference. The 3250 fps velocity that was posted earlier would work.

    Try some further distances to check your adjustments. I bet if you try using this much higher velocity you will find yourself getting much closer to your correct trajectory.
     
  10. Round2it

    Round2it Member

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    I have checked and rechecked my components and data - I have not checked the assumed 100 yard target lately. Now that I think about it, the target was taken down for mowing then put back up, presumably at the same distance. I will check that when I go back out this week. I really hope that's it.
    Thanks!
     
  11. JackinSD

    JackinSD Well-Known Member

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    Yeah that might help. However, unless it was moved about 85 yards further away, which I'm sure you would notice, your velocity is still going to need to change drastically.