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BC (Ballistic Coefficient?)

Discussion in 'General Gun Discussions' started by Marvinash, Aug 9, 2011.

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  1. Marvinash

    Marvinash Member

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    I have a ballistics chart for virtual every common rifle round, in various bullet weights. There's initials for a lot of things that I understand, (I.e MRT = mid range trajectory, MPBR = max. point bullet range).
    But the first line is BC, which I understand from the text, must mean "Ballistic Coefficient". A 55gr. .223 is .235 @ 3,000fps., a .308 averages .410 in three common weights at about 2,600fps.
    What is Ballistic Coefficient? What is it measuring?
     
  2. Johnny_Come_Lately

    Johnny_Come_Lately Member

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  3. olafhardtB

    olafhardtB Member

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    B C

    Imho bc=bs
     
  4. THplanes

    THplanes Member

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    Short version is it combines drag, sectional density, and bullet shape into one number. The higher the number the slower the bullet will lose velocity.
     
  5. Canuck-IL

    Canuck-IL Member

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    Try shooting a few different bullet shapes at 600 yds and you'll learn better.
    /Bryan
     
  6. MtnCreek

    MtnCreek Member

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    Physics disagrees with you…
     
  7. griff383

    griff383 Member

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    Ditto, at a few hundred yards most variables are negligable. Stretch it out some and youll notice real quick little things matter. Being off by as much as .5" at 100 yards will grow expoentially the further out you go and once you get way out you may not even be in the same area code.
     
  8. Mike1234567

    Mike1234567 member

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    "bc=bs" = BS;)
     
  9. BearGriz

    BearGriz Member

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    Where does one procure said charts? Do they take into account the barrel lengths?
     
  10. Zak Smith

    Zak Smith Moderator Emeritus

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    You can use printed charts, but it's more productive and instructive to use one of the online/free ballistic calculators such as JBM Online.
     
  11. Marvinash

    Marvinash Member

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    Thank you, Johnny-Come-Lately

    If I understand the links (and I think I do), BC could be 'dumbed down' to this: if a .308 has a BC of .435, that's about 125% of the bullet diameter (and 125% of the weight).
    Prior to it's MRT, (half-way to its MRT) a .308 has about the same energy as though it was a .435 weighing 200 gr., if the proj. is 150 gr. (bullet wt X 125%).
    At the equal distance PAST the MRT the factor for energy would be a corresponding .75% of .308 / 150 gr or the energy of a .210 weighing 118 gr.
    I would guess, that for a .308, that it would mean it would have that energy at about 500 yds. (I don't have the chart in front of me - I DO, but I would have to go to another window - THR would probably time-out).

    I don't understand the significance of barrel length on a traveling projectile once it leaves the gun. The bullet has no memory: if a .223 leaves the barrel at 3,000fps., given its twist, the barrel length no longer affects the behavior of the round. It affected it in giving it it's muzzle velocity; if it took 16" or 23" that's history.
    I don't know, but if a given bullet wt reaches a speed due to barrel length, would the round, at it's given speed vary from a longer barrel that produced the same speed? If the shorter barrel had a tighter twist, wouldn't it produce more stability - hence, better accuracy at a greater distance?

    My 'dumbed-down' explanation seems to crunch the numbers. If it is way off as a definition of BC, please (you who seem to understand this equation) tell me why I'm wrong in the simplest terms you can better use.
     
  12. Zak Smith

    Zak Smith Moderator Emeritus

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    BC is solely a ratio of the Cd curve for the bullet in question compared to the Cd curve for the "standard" bullet for the BC model in question. The two most common BC models are G1 and G7. Cd is the coefficient of drag. That's it.
     
  13. brickeyee

    brickeyee Member

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    BC is the ratio of any particular bullet to one of the 'standard bullets' used to make up the ballistic tables.

    If it was 1.000 you would be firing the exact same bullet that was used to create the tables.

    Any other bullet shape or size from the one used to create the table is going to require that the BC be used to adjust the table to match the 'non-standard' projectile.
     
  14. MtnCreek

    MtnCreek Member

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    Here's a dumb question: I'm not fully up to speed on BC and the history behind it, but why do most modern projectiles have such a low BC? The standard bearer for BC is a WWI bullet, correct? Most mordern bullets have a BC well below 0.50 and most LR target bullets range between that and 0.60. If someone knows how and why we went from 1.00 to avg 0.25 and now our best target bullets are somewhere around 0.60, please let me know. I assume it has something to do with a tradeoff between high BC and low enough bullet weight, but you know what they say about assuming...

    Thanks!
     
  15. Zak Smith

    Zak Smith Moderator Emeritus

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    http://en.wikipedia.org/wiki/External_ballistics
     
  16. brickeyee

    brickeyee Member

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    Zak got it.


    The 'model' projectiles are very large compared to what is routinely used in modern rifles.

    Part of it was probably the need to locate the projectile in flight accurately.

    A lot of paper targets at known spacing was an early method of receording the projectile travel.

    Now we can do it with sophisticated radar, but you stil lneed a decent size projectile or a very high frequency radar.

    The item being tracked needs to be multiple wavelengths of the radar long for the best accuracy.
     
  17. MistWolf

    MistWolf Member

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    Ballistic coefficient is a measurement of the bullet's drag, nothing more. BC can change with conditions such as altitude density and velocity. A rifle bullet with a high BC will lose velocity at a slower rate than a bullet with a lower BC launched at the same velocity and will stay supersonic out to a longer range
     
  18. Zak Smith

    Zak Smith Moderator Emeritus

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    You're rolling too much into BC. Cd is a function of mach number, and BC is a ratio, that can include fudge factors, that relates "this bullet's" Cd curve to the standard G1/G7/etc Cd curve.

    156_5700_img.jpg

    Here is a thread that goes into it
    http://thefiringline.com/forums/showthread.php?t=414392
     
  19. Jon_Snow

    Jon_Snow Member

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    Not true, it also factors in the weight of the projectile, which is very important in determining what downrange performance will be. If it didn't a 168 gr SMK would have a better BC than a 180 gr SMK, since it does have lower drag. The 180 gr tends to retain velocity better at longer ranges because the increased mass outweighs the slight increase in skin friction drag.
     
  20. Marvinash

    Marvinash Member

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    Good lord...!
     
  21. brickeyee

    brickeyee Member

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    No.

    It is the ratio of the bullets performance to the one of the standard projectiles.

    The standard projectile model already includes the drag of the standard projectile.

    All models are wrong. Some models are useful.
     
  22. MistWolf

    MistWolf Member

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    Sierra states that the BC can change with altitude density and velocity. They also measure the BC of bullets in flight using a doppler radar to determine how fast the bullet decelerates from drag
     
  23. Zak Smith

    Zak Smith Moderator Emeritus

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    The BC is a function of the speed of sound because it is based on mach transition not sped transitions. The speed dependence of BC is for when the profile of the bullets do not match close enough the profile of the base G1/G7/etc projectile. It's essentially a "fixup" to the piecewise math function that implements the curve I posted earlier.
     
  24. Heretic

    Heretic Member

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    Oh hell, where's my calculator?
     
  25. MtnCreek

    MtnCreek Member

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    Mr. Smith,
    Thanks for the good info,
    MtnCreek
     
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