Need help with "true velocity" for a drop chart

Discussion in 'Long Range Hunting & Shooting' started by kudu, Jul 29, 2011.

  1. kudu

    kudu Active Member

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    I'm trying to figure out a drop chart and need to know what the true velocity of my load is. I am trying to do this without a chronograph ( I have a Oehler 35 but I do not trust its numbers).
    I'm shooting a 7mm RM. loaded with 180 grain Bergers (stated BC is .672), zeroed at 200 yards. Went shooting this morning to recheck my numbers as the ones I got last week gave me two vastly different calculated true velocities and I figured I must have done something wrong. I ended up with very similar results today. Conditions Wind none, temp 65, pressure 25.20, elevation 4950.
    465 yards 5.1" high (5 shot group) using the G7 program the velocity is calculated at 2915. I then move to 753 yards and shoot a group 2.8" low the calculated velocity is 2686. Nearly 250 fps different. Same load, same gun, same shooter. I checked the distances with two different range finders multiple times. These data are very close to last weeks numbers so they are reproducible but they close are far numbers do not correlate. Any thoughts?
     
  2. Greyfox

    Greyfox Well-Known Member

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    This might be a dumb question. Are you using the g7, ballistic coefficient of the bullet iin a G7 calibrated program. If you use G7 BC 's with a G1 program you will get screwy velocities. The value. you site appears to be a G1 value. The G7 value is a lower value. I tried what your doing and was able to get pretty close. While it is claimed that the G7 is a better formula, I was able to match up very close to my chronograph using G1. I also have trouble trusting or relying only on my chronograph.
     

  3. Ridge Runner

    Ridge Runner Well-Known Member

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    agreed, no way the G7 BC for a 180 berger is that high
    RR
     
  4. kudu

    kudu Active Member

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    no i'm using the G1 BC. Just using the "ballistic calculator powered by G7" sorry for the confusion.
     
  5. joseph

    joseph Well-Known Member

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  6. kudu

    kudu Active Member

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    I understand knowing your gun, but mathematically there must be something I'm doing wrong here.
    I doesn't seem practical to shoot a drop chart at every possible elevation combination that I might have (1000 ft for Alaska moose, 2500 ft for Arizona Deer, 5000 ft for Idaho deer, 6500 ft for Wyoming antelope and 8000 ft for elk, then compound that with various temperature combinations that I might experience. Even if I was able to do that my barrel would be ready to be replaced by the time I was finished.
     
  7. bigngreen

    bigngreen Well-Known Member

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    Have you checked your optic for movement value? Are there terrain changes your shooting over or shooting at an angle?
     
  8. kudu

    kudu Active Member

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    I hadn't checked the scope movement but I just did that and it is right on (moved 25 inches with 25 MOA movement on the scope at 100 yards).
    The terrain is flat (some hills around but actually shooting level). No wind to push the bullet up or down.
     
  9. bigngreen

    bigngreen Well-Known Member

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    If you check your optic for like 20 MOA and it moves right on at 20 inches at 100 then your moving in IPHY not MOA which will catch you once you get out there and you will shoot low, just a possibility that has to be covered.
     
  10. kudu

    kudu Active Member

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    correct. I just learned that little piece of information. It should have moved 26.175" not 25". I reran the numbers but I still have a calculated velocity difference of about 200 fps. The close range gives me 2872, the far range gives me 2695.
     
  11. Broz

    Broz Well-Known Member

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    What scope? Sounds like a tracking issue to me.

    Jeff
     
  12. Broz

    Broz Well-Known Member

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    Ok,

    200 yard zero,

    180 Berger

    Conditions Wind none, temp 65, pressure 25.20, elevation 4950.


    Scope height above bore?

    Give us your actual needed dial ups you have confirmed and their exact distances. I assume you shot at least 3 shots to comfirm each?

    Jeff
     
    Last edited: Jul 31, 2011
  13. kudu

    kudu Active Member

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    scope height 1.75" (leupold VX 3, 4.5-14)

    zero 200 yards
    465 yards, 27 clicks (6.75 MOA) 5 shot group, 5.1 " high
    753 yards, 64 clicks (16 MOA) 5 shot group 2.8" low
     
  14. Broz

    Broz Well-Known Member

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    No rail base? The scope height numer looks low.

    I will run some numbers and report back.

    Jeff