BC issues

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by BB28MX, Jun 13, 2012.

  1. BB28MX

    BB28MX Well-Known Member

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    After shooting a chrono today and getting an MV of 2676 with 168 A-Max out of a 308 20 inch barrel with ES of 8 I am having troubles matching my drops with this info. My 680 yrd hold (not dialing and the scope at 12 power as it should to get accurate MILs) is roughly 3.8 MILs and my 1120 is almost exactally 8. In order to get this data from a program I have to use a g7 bc of .975! I'm at 3190 elevation, temp was 24.9 C, pressure was 29.67, and humidity was 18.7. Any ideas where I am going wrong?
     
  2. Jordan Smith

    Jordan Smith Well-Known Member

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    I'm guessing that your reticle spacing was not in true mils. Assuming a 200 yard zero, your 680 yard dope should be about a full mil more than what you reported, and the 1120 yard drops should be about 4 mil more! That leads me to believe that you need to zoom the scope in a bit in order to get true mils.

    A non-zero shooting angle at 1120 yards, etc, if not accounted for, may induce a little bit of error, as well.
     

  3. rscott5028

    rscott5028 Well-Known Member

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    You indicated 12x to get "true MILs". Is that Mfg specs, or did you measure/calibrate it yourself?

    What is your zero distance and scope height?

    Did you zero the rifle and shoot your long range from the same position?

    How many did you fire at each distance?

    How did they group for each distance?

    How did the measured velocity of each long range shot compare to the 2676 used for computing hold over?

    -- richard
     
  4. BB28MX

    BB28MX Well-Known Member

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    Ok so I figured it out... Target was at 605 not 685, and a more realistic g7 around .310 gives me correct holds for that distance. Guess I had too many numbers in my head but thanks for the help!
     
  5. Bullet bumper

    Bullet bumper Well-Known Member

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    A JBM conversion of the G1 BC of .475 ( 168 A Max From Hornady site) . 168 grain , 2676 fps , to a G7 Bc = .239 G7
     
  6. BB28MX

    BB28MX Well-Known Member

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    I just used that g7 bc and got 4.33 MILs for a hold but I was holding 3.8 and hitting fist sized rocks at what I have now confirmed to be 600. I could try to retest the power at which MILs are accurate. Is there anything else I should retest?
     
  7. Bullet bumper

    Bullet bumper Well-Known Member

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    One single BC at one velocity can not model all that well compared to several BC's and velocities and also the trajectory is affected by local atmospheric and elevation conditions and angle of fire and cant .
    The difference of .5 mill between real world drops and a ballistic program could be a result of some of the above changing .
    Enter the actual measured angle of the barrel above horizontal the elevation above sea level , the temp & humidity at the time of shooting and see if that is a closer match to what drop you are seeing on the targets.
    1 mill = 3.6 inch at 100 yards . So you could mark a target with that inch measurement and place it at exactly 100 yards from the scope sight . Then see at what power it corresponds to 1 mil on the reticule . If the scope is a second focal plane that is. If it is a FFP then it should be correct at all powers as the reticule grows with the power increase .
    .5 mil is about 10.8 inch at 600 yards so make sure you are able to see that difference and aim with precision at 600 when shooting at rocks . Maybe a paper target may give more precision as to exactly what drop you are seeing as you can measure from the center of the group .
    Fist size rocks at 600 yards would be around .5 MOA accuarcy that is very good.
     
  8. rscott5028

    rscott5028 Well-Known Member

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    One or two long range shots at rocks isn't enough information to question or overturn the manufacturer's published BC.

    ...especially if you're calling it 685 yds and it turns out to be 605 and then 600 even.

    You've got to get very tight with the details if you want a proven/predictable load.

    Put some groups on paper with enough shots and consistency to count.

    -- richard
     
  9. Broz

    Broz Well-Known Member

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    Most errors are in velocity from inaccurate chronographs or methods of use. Also, at that short of a distance holdover is fine but not what I feel is the most accurate method to shoot groups.

    As Richard stated many groups will need to be shot and also you will need to consider the error factor of the rifle , ammo and shooter combined.

    Jeff
     
  10. BB28MX

    BB28MX Well-Known Member

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    I have two targets set up at this range, one at 605 and one at 685. The rocks I was shooting were at 605 with a 3.8 mil hold. The milk jug was at 685 and required a 4.9 mil hold. If I replace the published bc with the bc I have come up with those dopes match perfectly. I do plan to reshoot to verify and add some farther targets to get more data as well as remeasure my scopes MILs but at this time I don't feel I'm jumping the gun to replace the published bc. Especially since if I use the published bc (as I did at the range) I am consistently high. At the range I had a 5 degree down slope that equates to so little when I used my angle adjustment calculations that I can all but ignore that part. I know this gun shoots .5 moa or better so I am ruling that out. All I keep coming up with is the bc. Hope this clears a few things up and please let me know if I have over looked something or miscalculated. Thanks again guys
     
  11. Joe King

    Joe King Well-Known Member

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    One of the guys I work with ran into a similar problem with his DPMS panther, and 175gr SMK's. I showed him JBM ballistics calc using the bullets with Litz instead of the advertised bc. became much closer. He knew what his dope was already he just didn't understand the discrepancy until then.
     
  12. rscott5028

    rscott5028 Well-Known Member

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    Sounds like you have it under control.

    Like Jeff pointed out, incorrect MV is often the offender.

    While you can manipulate almost any variable e.g. MV, BC, Scope height, or whatever to match your dope, when conditions change, you will be off again at longer distances if you didn't manipulate the correct variable.

    Many times it comes down to a stacking issue where several small things are off by a little, but add up to a noticeable difference.

    -- richard
     
  13. Broz

    Broz Well-Known Member

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    If you are shooting with hopes of calculating accurate drops, I strongly suggest you make your targets from paper, cardboard or wood with a very small point of aim to reduce as much error as possible. This way you have something accurate to measure to center of group from point of aim. The more groups the better.

    Jeff
     
  14. BB28MX

    BB28MX Well-Known Member

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    If I used litz's bc of .230 I get 4.41 for 600 and 5.4 for 680... That's a huge difference from what I was shooting. If that bc is accurate something else must be off quite a bit. I might see if I can get a group at 800-900 and see if the velocity is off or if ( for reasons unknown) I should use a bc different than litz's