Ballistics Question

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by barnesuser28, May 18, 2012.

  1. barnesuser28

    barnesuser28 Well-Known Member

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    Ok would it work better to adjust your BC until it matches up with your actual drop? or would it work better to adjust mv until it matches up with the drop
     
  2. Greyfox

    Greyfox Well-Known Member

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    If my BC data is from a reliable source, ie. Bryan Litz, I will initially adjust my velocity input in my calculator to see if I can match my actual drops. I have had good success using this approach. If I can't get my drop data to line up with my selected distances changing my velocity input, I will adjust BC, using my chonographed velocity input until it lines up.
     

  3. barnesuser28

    barnesuser28 Well-Known Member

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    so in your experience are Barnes's listed BC's close to their actual BC
     
  4. Greyfox

    Greyfox Well-Known Member

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    I don't know. I have never used them. Bryan Litz has run BC's on the Barnes bullets and has them listed in his book. I use Bergers, Sierra, Nosler,and Hornady. Bergers and Sierras listed BC's are close. I use Bergers mostly for hunting and Sierras for target and competition.
     
  5. Mikecr

    Mikecr Well-Known Member

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    I say it's best to figure out which input you're supplying that's wrong.
    You should know your MVs. You should know your scope height, and atmospheric conditions, and ranges. You should've tested the scope to find out the actual adjustment values.
    Rule out coriolis, and spin drift..

    With this, and considering the gun's accuracy capability, you would tweak the BC (possibly switching to G7 BC entries).
     
  6. barnesuser28

    barnesuser28 Well-Known Member

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    How do you do that
     
  7. Bart B

    Bart B Well-Known Member

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    Clamp your scope solidly with it at maximum power aimed and focused at a ruler exactly 50 yards away from the front of the scope and at exact right angles to the line of sight. Make sure the ruler's aligned straight with a reticule wire. Align the reticule on an even inch line. Count the clicks as the reticule moves to another inch line 2 inches away.

    If it takes 17 clicks to move the reticule exactly 2 inches at 50 yards, each click's worth .2353 MOA. Check both elevation and windage to verify they're both the same. If 17 clicks don't put the reticule exactly on the ruler 2 inches away, keep moving the adjustment in the same direction until it aligns exactly on an inch line; counting clicks all the way. Then divide the inches moved by the clicks to get there. 1 MOA in the shooting sports is exactly 1/2 inch at 50 yards.

    You'll get a bit more accurate results if the ruler's at 100 yards.
     
  8. barnesuser28

    barnesuser28 Well-Known Member

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    So how do you then enter that into your ballistic calculator. I have shooter
     
  9. Bart B

    Bart B Well-Known Member

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    I have no idea whatsoever. Read the software's instructions. If it doesn't explain how, here's what you can do.

    Let's say your software says a given bullet drops 300 inches at 800 yards for a given muzzle velocity and atmospheric conditions. After getting a good zero at 100 yards, add the bullet's drop at 100 yards (from the software) to the scope height above the bore; let's say it's 3 inches. Adjust the scope's elevation down whatever it takes to move the reticule exactly 3 inches down. Now your scope's "ballistically" bore sighted to the rifle.

    Next, move the elevation adjustment the number of clicks to move impact up 303 inches at 800 yards. If one click's worth .2353 MOA, you'll need to come up 37.875 MOA or 161 clicks. Each click moves impact 1.8825 inch, 303 divided by 1.8825 is 161.

    Go shoot at 800 yards. If your not right on in elevation for the atmospheric conditions at the range, one or more of the following's not good:

    * The actual range to the target

    * Ballistic software

    * BC for the bullet

    * Muzzle velocity

    * Atmospheric conditions entered

    I've used Sierra Bullets' software for .308 Win. .30-06, and two 30 caliber magnums with different bullet weights and velocities on rifles with sights having ajustments per click of .2 MOA to .6 MOA testing at 600 and 1000 yards. Calculating sight settings using the above method's put first shot within 1/2 MOA of perfect elevation for each. I think the reason is Sierra's software uses 3 to 5 BC's for their bullets that's been determined by actual firing tests measuring time of flight between two points. Shooter software uses only 1 BC for each bullet and I don't think that's going to produce data as reliable as software using BC's the bullet actually has at different speeds through the air.
     
  10. Mikecr

    Mikecr Well-Known Member

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    Depends on your ballistic calculator, and what your scope ends up adjusting at.
    If it is true MOA (1.0472 IPHY) you would select appropriate MOA value per click(1,1/2,1/4, 1/8).
    If it's inches per 100yds(IPHY) you should be able to enter the specific value of your adjustment.

    If your scope is in MOA, you can dial up 10moa, and this should take you up just a shade(~1/32) under 10.5 IPHY. But don't over generalize this, or assume anything. Most scopes are not true MOA..
    I've seen NF NXS scopes that are not(1.09 IPHY), and so far the only scopes I've tested that turned out true MOA have been Leupold's Mk4 scopes.

    This can be a significant source of error for single shots in the field.
    If you need 10moa to hit your mark @ 500yds, and you have your software set in MOA, but your scope is actually IPHY, then you could be off ~1/2" per 100yds, or 2.5" at 500yds(just in adjustment error).
    At 1kyd, and 30moa needed adjustment, you'd introduce over 14" of error.
    This may be why holding off with moa reticles seems popular, as 30moa optical hold-off is just that.
    But provided your adjustments are determined and properly accounted for, dialing is more precise than optical hold-offs(e.g. between even hash marks, +/- subtension errors).
     
  11. barnesuser28

    barnesuser28 Well-Known Member

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    thats what i have a leupold mark 4, so you are saying if it is not a true 1/4 moa adjustment .2618 then i should use the iphy setting on my ballistic calculator for whatever the actual adjustment is
     
  12. Bart B

    Bart B Well-Known Member

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    According to Shooter's website info on their software, it supports MOA, IPHY and Mils.

    But in my tests of various scopes on a bench collimator accurately measuring adjustment values per click, they vary up to 5% or more. Even the same model for a given make their optical tolerances in focal length can add up to a 5% spread. If the focal length of a set of lenses is 5% more than spec (normal high end optical lens tolerance, both camera and telescope lenses), focusing the target on the reticule makes the image 5% bigger than the adjustment mechanics are set for. That ends up making the adjustments 5% smaller per click; 0.2375 instead of 0.2500 unit if the clicks are 1/4 unit per click. The mechanics of scope sight adjustments are set as close to the spec as manufacturing allows. Once set up, they cannot compensate for the tolerances in the scope's front end objective lens group tolerances.

    If you want near zero tolerance in your numbers, better measure your scope's adjustments to the same accuracy.

    There's about 4.7% difference between trig MOA's and shooting MOA's (inches per hundred yards). And a scope spec'd at 1/4 iphy per click may actually have 1/4 trig MOA.
     
  13. Mikecr

    Mikecr Well-Known Member

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    Yes
     
  14. barnesuser28

    barnesuser28 Well-Known Member

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    ok thank you