You Only Need to Know This If You Shoot Past 700 Yards!

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Eaglet, Feb 16, 2013.

  1. Eaglet

    Eaglet Well-Known Member

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    Using The Right BC in Your Ballistic Application

    A bullet has two (2) different BCs
    values under the same drag function.

    Remember, ballistic coefficient depends on
    Air Density.

    BC (Ballistics Coefficients) are either based
    on "US Army" also known as "Standard
    Metro" or on ICAO Standard Atmosphere
    adopted by the US department of Defense in
    1956. Here is where the two different BC values
    come in.

    Here is the catch. You need to know what
    Standard Atmosphere your Ballistics
    Application is referencing the BC on. Said
    in different words, we need to know which of
    the two BCs our ballistic application is
    expecting us to provide, either the one
    based on ICAO or the one based on Standard
    Metro.

    Under about 700 yards you don't have to
    worrie about these differences, but other
    wise we should probably be aware of them.

    Example:

    As most of you know, my favorite Ballistics
    Application is ColdBore 1.0; this application
    assumes that the BC that I'm entering in is
    referenced to the ICAO Standard
    Atmosphere.


    BC Reference
    Berger ICAO
    Barnes Std Metro
    Hornady Std Metro
    Nosler ICAO
    Lapua ICAO
    Sierra Std Metro
    GI APG ICAO

    So, Berger and Nosler and Lapua bullet manufacturers their BC values are referenced to ICAO and that’s exactly the value that my application expects me to give it.

    If, I use Hornady bullets with ColdBore, I need to modify their BC which is referenced to Std Metro to ICAO. To do this all I have to do is multiply the given BC by 0.982
    The 0.338 Barnes 265 gr. Bullets is claimed to have a BC of 0.575; so I would modify it by:
    0.575 x 0.982 = 0.565, and that would be my BC value to use in CB1.0

    There it is, for this bullet in the G1 drag function you have two (2) different BC values, 0.575 and 0.565, we also know which one to use in this particular case.

    Now, if the application is to use BC based on US Army “Metro Std”, then you would use Barnes given BC value with no modification.

    Ballistics Explorer is one of the so many applications using the Metro Std. In this case, to use an ICAO based BC value, you would modify it by multiplying it by 1.018…
    0.565 x 1.018 = 0.575-------

    The difference can make 1.5 FT difference at 1500 yards…

    How do I know what my Ballistics Calculator or Application uses?

    Usually, when you run a Ballistics Application you would be able to see the input boxes populated by default values. Look at the atmospheric STD conditions and if you see this:

    Altitude: 0 ft.
    BP of 29.53 inches of mercury at a
    Temperature of 59 degrees F
    Humidity = 78 %

    Then you know the application assumes the user to give it BC values based on “Army Standard Metro Conditions”

    If you see this:

    Altitude: 0 ft.
    BP of 29.92 inches of mercury at a
    Temperature of 59 degrees F
    Humidity = 0 %

    Then you know the application assumes the user to give it BC values based on ICAO

    Good Shooting!

    Note: Most of the information I got from CB1.0 User Manual. :D
     
  2. tomt

    tomt Well-Known Member

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    Very good, thanks!!
     

  3. Capt Academy

    Capt Academy Well-Known Member

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    Wow! I did not know this, thank you sir!
     
  4. 7stw

    7stw Well-Known Member

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    Interesting stuff, and easy to see how you COULD make a mistake on the calculation, when entering in the PROVIDED BC info. Thanks for the update!
     
  5. bigbuck

    bigbuck Well-Known Member

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    I use NF Ballistic software or perry systems. I shoot the 210 Berger and I enter Bergers Advertised BC with my NF . Do you know if this is correct ? Thanks.
     
  6. Joe King

    Joe King Well-Known Member

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    Your talking only about G1 BC figures correct?

    Myself I use G7 and to my understanding it is for all intents and purposes a tiger of a different strip, but basically a better fit (base model) to modern long range bullets.

    I'm not trying to hijack or confuse anyone so I think it should also be pointed out that.
    The standard model projectile for a G1 BC is a flat based spire point

    Where the standard model for G7 is a Boattail spire point.

    Maybe Bryan Litz can weigh in on this, since he's my source of info on this matter.
     
  7. Eaglet

    Eaglet Well-Known Member

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    Friend, I have Exbal v6.5 and it requires BC values based on "Metro Std".
    You need to modify the BC values from Berger and Nosler, but you don't from Sierra.

    At the risk of repeating my self, any ICAO based BC values you modify by multiplying
    by 1.018 which of course will produce a higher BC value.
     
  8. Eaglet

    Eaglet Well-Known Member

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    Joe, your original question is very good and valid. I'm glad you asked.

    Remember that this topic is not about G1 vs. G7, in fact, it has nothing to do
    with that. This is all about Air Density and how BCs are referenced to.

    Now, the correction is the same no matter what Drag Function. G1, G5, G6, G7 etc.

    Just making sure I explain my self, I'll give an example:

    The ICAO referenced G7 BC for Berger 300 gr Match Hybrid OTM Tactical is: 0.419--
    to use this value in "Exbal" or "Ballistic Explorer", you would modify it to 0.419X1.018=0.427
     
  9. elkaholic

    elkaholic Well-Known Member

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    Thanks Eaglet, this is very good info that I wasn't aware of.....Rich
     
  10. jkupper

    jkupper Well-Known Member

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    Okay. For the newby here, could you explain how much difference it would make to use the wrong B.C. For example, let's just say a 7mm Berger with a B.C. of .617, like the 168 grain VLD :D
     
  11. Eaglet

    Eaglet Well-Known Member

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    jkupper, since you did not specify a muzzle velocity etc. let's say this is done at
    2900 f/sec muzzle velocity, at sea level and std conditions.

    CaptureDiff.JPG
     
  12. bigbuck

    bigbuck Well-Known Member

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    Thank you Sir. I am all about gtting the most accurate results .:)
     
  13. LouBoyd

    LouBoyd Well-Known Member

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    If you want the most accurate results arguably you shoudn't be using BC's at all. For most low drag bullets the G7 is better than G1, but there are still significant differences between the predictions and reality. Look at Brian Litz's book in the back chapters where the match between G1 and G7 trajectories are compared to millimeter radar measurements of the actual velocity vs air density results. In most cases the G7 is a better match for "long range" bullets. There are some notable examples where the G1 is a better match. The 240 grain Sierra Matchking is an example. It's really not a VLD design even though it's a spitzer boatail.

    Most of us don't have access to a range equipped with mm doppler radar and I don't know of any available ballistics programs which allow entering doppler radar results directly even if you have them.

    You do have the option of doing your own shooting tests from your rifle and cartridges. It's more tedious than just relying on a computer output but you can use most modern ballistics programs to "curve fit" measured results to give better results than you're likely to get using manufacturers published G1 or G7 numbers. By better results I mean a better prediction of shot placement at various ranges using your handloads (or specific model of quality factory ammo). and your rifle in your environment. You still have to adjust for your local air density at the time you shoot and (perhaps) the temperature of your ammo at the time of firing.

    Few shooters want to take the time and effort to get the most accurate results for "first shot" accuracy. For target shooter who are allowed sighters knowing the exact BC and exact air density makes little difference. For hunters, snipers, and some competition with targets requiring first shot hits at random targets and random ranges it may be worth the effort.

    I have far more confidence in a set of range cards set up for my rifle and specific ammo for a range of air density derived from shooting tests made in the environment where I shoot than any computer generated results based on factory numbers for BC. You can make a good table over the supersonic range of a rifle with about 20 carefully fired shots. Certainly using G() functions and a ballistic calculator are useful just to put the shots on a target face so you can measure them. Wind deflection can be calculated from accurate drop measurements. Trying to measure wind deflection vs natural wind vectors in taking the drop measurements for the range card. Shooting over flat land in low wind will result in better quality drop measurements.
     
    Last edited: Feb 17, 2013
  14. Red hunter

    Red hunter Well-Known Member

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    A couple weeks ago I was reading a thread comparing the 210 and the 200 Berger bullets with plenty of first shot kills over varying yardages. In this reading I believe the writer stated he put in 777 yards and there was a 10mph 3o'clock wind plus other data and was given a shooting dope from his ballistic calculator. I believe he stated this gave him about a 24 inch wind drift. I input this in strelock and came up with 17 inches or so. I chalked up the difference to a phone app and a high dollar ballistic calculator. Who am I to argue with the result of a one shot kill at that range. I tried several altitude adjustments and shooting angles and never came close. I figured the best results are always shooting cards for me. At least till I can afford the real deal.