Ballistic program not matching actual

Discussion in 'Long Range Hunting & Shooting' started by caryo15, Oct 26, 2011.

  1. caryo15

    caryo15 Member

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    I'm new at the long range shooting (past 600). I've not posted but have researched here alot. My exbal program is not computing the correct drop and wind corrections. I use a kestrel 4000 and even took my chronograph last time, recording data on every shot. I have data on 2 rifles and both need more MOA than the exbal program is telling me. The difference is greater with distance, with approximatly 4 moa difference at 1000 yards. I've tried to plug in different variables into the program but cannot get it correct.
    Is there any common mistakes that might be the problem? Any help would be appreciated.

    Thanks,
    Cary
     
  2. bigngreen

    bigngreen Well-Known Member

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    Post up a little more info, on the rifle and the inputs you've been using, it will help! Have you done a trajectory validation?
     

  3. caryo15

    caryo15 Member

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    Lots of data: 308 DPMS, 168 SMK, 2806-2875fps through out the day, 1280', avg. 83 F, avg. BP 30.07, avg. humidity 48.9%

    first moa is actual hold - where it impacted, adjustment the computer called for

    100: zero
    200: 1.75 moa - center, comput: 1.5 moa
    300: 4.0 moa - 2" low, comput: 4.0
    400: 7.0 moa - center, comput: 6.75
    500: 10.0 moa - 3" low, comput: 9.75
    600: 14.0 moa - 2.5" low, comput: 13.25
    700: 19.0 moa - 3" low, comput: 17.25
    800: 25.0 moa - 3" low, comput: 21.75
    900: 30.75 moa - center, comput: 27.0
    1000: 38.5 moa - 6" high, comput: 32.75

    Thanks,
    Cary
     
  4. caryo15

    caryo15 Member

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    Yes, i did a trajectory validation. It helped but not alot. It would get the longer ranges closer but make the near ranges further off.

    Cary
     
  5. eddybo

    eddybo Well-Known Member

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    What scope are you using? Are you sure you are dialing MOA vs IPHY?

    Did you measure your scope height above the bore?

    Maybe a lot of little errors compounding?
     
  6. caryo15

    caryo15 Member

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    leupold LR with target turrets, 1.6" above bore.
     
  7. SBruce

    SBruce Well-Known Member

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    Assuming all the data is input correctly, I'd try a tracking evaluation on the scopes adjustments. Just to make sure the adjustments are moving the reticle the advertised amounts.
     
  8. Nimrodmar10

    Nimrodmar10 Well-Known Member

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    You can figure that the advertised B.C. is correct, but even with a chronograph you can't be sure that your velocity is. Plug in different muzzle veolocities until the ballistics program matches the real drops match. That figure will be your true velocity. Don't necessarily believe your chronograph. If in doubt measure your vejocity at the muzzle and at 100 yards, preferably at the same time. Plug those figures into a B.C. computing program and if the B.C. doesn't match the advertised B.C. then your chronograph is probably off.
     
  9. twiedenh

    twiedenh Well-Known Member

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    What about some cant in the scope? Is it mounted perfectly?
     
  10. WildRose

    WildRose Well-Known Member

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    A slight cant to the scope would not effect the high/low much at all. Mostly that will screw up your windage adjustment.
     
  11. WildRose

    WildRose Well-Known Member

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    I used your figures and came up with the following. Average BC of .425, Average velocity of 2850fps.

    100-0.0 Moa
    200-1.7
    300- 4.1
    400- 7.0
    500-10.3
    600-14.1
    700-18.5
    800- 23.5
    900- 29.3
    1000- 36.0

    These were using average velocity of 2750fps

    0.0
    1.9
    4.5
    7.6
    11.2
    15.4
    20.1
    25.6
    31.9
    39.2

    That looks pretty darn close to your real impacts.

    I used Hornady's ballistic calculator.

    Ballistics Calculator - Hornady Manufacturing, Inc
     
    Last edited: Oct 27, 2011
  12. bigngreen

    bigngreen Well-Known Member

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    Anyone know the Litz BC for this bullet? I've got it very close but I don't buy the BC.
     
  13. WildRose

    WildRose Well-Known Member

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    Tweaking it a little more I got even closer to what your actual field data shows.

    100-0.0
    200-1.9
    300-4.6
    400-7.7
    500-11.3
    600-15.4
    700-20.0
    800-25.2
    900-31.2
    1000-38.0

    This was using a BC of .450 and 2,700FPS MV.

    Keep in mind that you will virtually never get your field results to be exactly reflective of what the programs spit out as there are simply too many variables that come into play due to the fact humans are involved.

    It looks like you are sub .5MOA at almost all ranges to 1000yds and that ain't bad.
     
  14. webs

    webs Well-Known Member

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    The Sierra matchking has a G7 of .218 and G1 of .427 according to Litz's book.