Are you thinking of BC the wrong way?

Discussion in 'ELR, Ballistics & Bullets Board' started by DocUSMCRetired, May 6, 2017.

  1. DocUSMCRetired

    DocUSMCRetired

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    Are you programmed to incorrectly picture what a BC should look like?

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    What do you think of when you think of a BC? Does a simple one number solution come to mind? Or do you picture a velocity band? When you talk about the BC of a bullet, it is important to make sure that you don’t forget the other half of that number. A BC should always be followed by an indicator. It is not just enough to call upon a single number. By using a single number you don’t see the entire performance of the bullet. You also don’t get a fair comparison of one product to another. Making it easy to push a little marketing hype in to what the consumer thinks. This can also hide a bullets true performance, and some bullets have a wide variation in BC from Mach 2.5+ to Mach 0.9-(Subsonic), while others are more consistent. Anytime you see a BC you should be asking yourself “at what velocity”. All BCs published by Applied Ballistics are averaged from 3000 fps to 1500 fps. Averaged is the key word here. It is important that the consumer understands comparing apples to apples. You cannot take a Mach 2.5 or high BC and compare it an averaged BC or a Mach 2.0 or Mach 0.9 BC etc.

    A BC is only true at 1 specific velocity, which is why you can find velocity bands published in the Ballistic Performance Book, but also why you can twist a little marketing hype in to them by using the highest performing velocity band as the published BC on the box. (You can hide a bullets flaws this way) As Bryan put it "The BC changes a lot over the flight of the bullet."
    As a shooter anytime you see a BC you should be seeing 2 indicators. Think of it like when you look at a cars mpg. The cars list “## highway, ## city”. Well a BC should say “## G7 Averaged” or “## @ Mach 2.5 & ## @ Mach 1.0 & etc. “. You could use ## G7 @ 2800 fps, and it would be just fine. As long as you know where in the velocity band that BC was calculated at, it allows for a more honest comparison against other bullets. So the next time you see a BC the first question that pops in to your head should be “where is the rest of the story” followed by asking the question “at what velocity”.

    Of course, if you use Applied Ballistics CDMs you don't have to worry about velocity effects of BC as your bullets drag is being modeled all together. Giving you not only a more accurate prediction, but also removing any marketing bias. However we know it can be hard to compare bullets on the shelf this way. So it is still important every time you see "BC G# 0.###" that you ask yourself "At what velocity".
     
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  2. steve_podleski

    steve_podleski

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    what is needed is something like a drag*weight v/s Mach curve to replace BC
     
  3. mikecr

    mikecr

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    I don't see where weight or sectional density (for BC) are needed at all.
     
  4. steve_podleski

    steve_podleski

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    Wind resistance is a function of weight and drag: F=ma
     
  5. damoncali

    damoncali Gold $$ Contributor

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    You need both to translate from BC to Cd.
     
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  6. Alan Warner

    Alan Warner

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    Doc:
    To answer your own “where is the rest of the story” --- All our data is verified with Doppler,( is that hype?) and there is not one of our pills launched at the same speed as ANY other out there of the same weight that comes anywhere near our retained speed at distance. Our rather lackluster 155 30 cal reaches 1000 yds with 300+ fps more retained velocity than the rest of the best, same weight bullets available. We will most likely revisit the 155/30cal design in the future and make it flatter yet than the 5 to 7 minutes it is now. I may be in error, but right now, WarnerTool
    has the only 375 pill that will maintain above mach 1 at 2 miles. How bout you?

    Alan Warner
     
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  7. mikecr

    mikecr

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    I should have said I don't see where BC, or weight or sectional density (for BC) are needed at all.
     
  8. damoncali

    damoncali Gold $$ Contributor

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    You need them in some form to calculate a trajectory.
     
  9. 300 RUM

    300 RUM Gene Nowaczyk

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    If anybody wants to send full dimensions of their bullet I can run a cfd on them
     
  10. steve_podleski

    steve_podleski

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    What code do you use?
     
  11. boltfluter

    boltfluter Gold $$ Contributor

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    Crickets? Sure got quiet from the other camp. Lol.

    Paul
     
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  12. steve_podleski

    steve_podleski

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    Is the Doppler effective to 1000yd?
     
  13. damoncali

    damoncali Gold $$ Contributor

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    Its really annoying how bullet manufacturers are so vague about what their reported numbers actually mean. Try comparing a Sierra bullet to, well, anything. It's great that they take the time to wring out the best accuracy from a ballistics calculator. I applaud that.

    But 9 times out of 10, when I'm looking at BCs, I want to know which bullet will deflect less at a certain range. It's painful to extract this information these days, and it's only getting worse with vague statements like "BC at velocity X" (Hornady does this with their ELD bullets). What velocity is that? Muzzle velocity? The exact velocity that the bullet happens to be going? Some range of velocities that averages to that velocity? It's getting less clear, not more clear.

    I guess that's the nature of competing products, but it sure would be nice to see a clearly defined standard of how BC is reported.

    I shouldn't need a ballistics calculator to compare a Berger hybrid to a Sierra MatchKing, and hope that the details were interpreted right.
     
  14. mikecr

    mikecr

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    No, things like weight & sectional density apply to internal and terminal ballistics, and stability, but not directly to external ballistics.

    None would be needed for solutions where the external ballistic software didn't require their input. If setup for direct lookup of drag from a developed table, you could drop BC, form factors, drag coefficients, the bullet all together from the math. Once you know it's drag, the bullet itself means nothing to solution.
     
  15. damoncali

    damoncali Gold $$ Contributor

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    That's not how it works. It is mathematically impossible to calculate the trajectory of a projectile without "F" (the drag force), and "m" (the weight). F = ma. The drag force comes from knowing the drag coefficient, velocity, cross-sectional area, air density and velocity. "m" comes from knowing the weight. You must have all of the above or you simply won't have enough information to work with.

    BC is just a clever way to combine drag coefficient, weight, and area into one number (along with a standard drag function, like a G1, which is itself just a list of drag coefficients at different velocities).

    You can skip directly inputting area (caliber), weight, and drag coefficient IF you instead use a BC and standard drag function. But it's all the same stuff, just rearranged. There are no short cuts, just conveniences.
     
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  16. mikecr

    mikecr

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    I think we're just talking about different points in the solution process, and what is -vs- what could be.
    My point is that whether predicting drag at mach# from parameters including projectile mass & shape, or simply deriving it using RADAR, once you have drag fully defined, then knowledge of the projectile is not needed for drop solutions. With RADAR it was never needed.
    At that point in the process, drag, like gravity, is simply what it is, and whether the projectile is a needle or a Volkswagon it will fall at the rate of gravity and displace an ever smaller horizontal distance(due to changing drag/velocities) over each increment of time. Slow here to consider that we're not predicting drag any longer -if it's already known and published. And removing so much prediction could lead to better solutions.
    The ballistic math might only need to be setup this way.
    If Berger does this for us,, it's big.
     
  17. damoncali

    damoncali Gold $$ Contributor

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    I see what you're getting at now. I think there are advantages to not lumping everything together - for instance, when comparing bullet designs in the abstract. It's nice to be able to separate out a drag coefficient, and better still if you can describe a bullet's Cd in terms of its geometry. Think about how great it would be if you could describe a bullet, and then accurately calculate it's aerodynamics without radar or other test data. There's no reason that this is inherently impossible. It's just not yet possible.
     
  18. IdahoSharpshooter

    IdahoSharpshooter Gold $$ Contributor

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    Alan,

    are you a company where bullets can be purchased? Got a website?

    I am curious, whose 375 caliber "pill" :rolleyes: won the King of Two Miles competition last year?

    Not to stir up things, but are you gentlemen shooting a 375 of some sort at these mile-plus matches this year (or 2016) with your bullets? I would love to see some competition results if you can provide them. The 375's seem to be THE caliber for serious long range these days...

    thanks,

    Rich
     
  19. JRS

    JRS Silver $$ Contributor

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    Surely you know who Alan Warner is, Rich.

    Warner Tool Company Inc.
     
  20. sparker

    sparker

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    I believe that those who simply look at BC as the primary determining factor for purchase are negating the most important consideration which is accuracy. Most bullets of similar design within a given weight class have quite close BC values. The need to optimize consistent accuracy far outweighs the need to optimize BC. I do find it interesting that this talk about "marketing hype" ignores the company that began the BC marketing hype.

    Establish an accurate load, verify trajectories at range. All else is moot.

    Scott Parker
    Single Kernel Scales
     

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