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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
ballistic coefficients?
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<blockquote data-quote="MAX" data-source="post: 27968" data-attributes="member: 184"><p>I'll give it a try Brent, but one thing for certain, I ain't no Robert McCoy!</p><p></p><p>BC evaluates the relationship between weight, form and diameter. It is a ratio with no dimension, a comparison of "X" to a standard(G something) with known ballistic characteristics. Because the factors relating to the calculation have direct relationships, ie. BC is proportional to SD for a given form, inversely proportional to diameter for given form and weight, etc. etc. It is fairly convenient to use(remember, figures don't lie, but liars do...), but it's accuracy is dubious(garbage in-garbage out) in field conditions.</p><p></p><p> The high priests of modern ballistics began to look at this issue in a strictly aerodynamic sense. What matters to them is COEFFICIENT OF DRAG. It matters not what the size in weight or diameter(in most cases). The Cd is purely a function of form, and it is expressed in Mach Numbers. One of the points of this is that Mach is Mach, while FPS may be one Mach # today, and another tomorrow. The drag curve, when plotted looks pretty much the same, but when you ID a point where it says Cd=.23 at Mach 3.8 it is a reliable fact. Doesn't matter if it's -20 or 120* F, it's still a Cd of .23 at Mach 3.8 </p><p></p><p>Anyway, after all of this, additional factors are applied to evaluate what the projectile will do, items such as weight, GS, etc., and perhaps a big reason the industry hasn't embraced this concept is that a) they have a system in place and it's comfortably slippery. b) the equations used in the Aerodynamic approach have names like "differential" and "quadratic", and it's really neat if you have a Cray handy to crunch your numbers! Too, if you want the true Cd for a given form, you have to test it. Extrapolation is fuzzy science.</p><p></p><p>Anyway, that's my take on this FWIW, and I'm sure I need to read it a few more times.</p><p></p><p>"Modern Exterior Ballistics", by Robert McCoy </p><p></p><p>If you happen to buy this book, take a good look at the depicted/graphed Cd values as speed increases beyond the Mach 4 regions. It makes my madness seem a little less severe, my personal musings more worthy. <img src="http://images/icons/wink.gif" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p></blockquote><p></p>
[QUOTE="MAX, post: 27968, member: 184"] I'll give it a try Brent, but one thing for certain, I ain't no Robert McCoy! BC evaluates the relationship between weight, form and diameter. It is a ratio with no dimension, a comparison of "X" to a standard(G something) with known ballistic characteristics. Because the factors relating to the calculation have direct relationships, ie. BC is proportional to SD for a given form, inversely proportional to diameter for given form and weight, etc. etc. It is fairly convenient to use(remember, figures don't lie, but liars do...), but it's accuracy is dubious(garbage in-garbage out) in field conditions. The high priests of modern ballistics began to look at this issue in a strictly aerodynamic sense. What matters to them is COEFFICIENT OF DRAG. It matters not what the size in weight or diameter(in most cases). The Cd is purely a function of form, and it is expressed in Mach Numbers. One of the points of this is that Mach is Mach, while FPS may be one Mach # today, and another tomorrow. The drag curve, when plotted looks pretty much the same, but when you ID a point where it says Cd=.23 at Mach 3.8 it is a reliable fact. Doesn't matter if it's -20 or 120* F, it's still a Cd of .23 at Mach 3.8 Anyway, after all of this, additional factors are applied to evaluate what the projectile will do, items such as weight, GS, etc., and perhaps a big reason the industry hasn't embraced this concept is that a) they have a system in place and it's comfortably slippery. b) the equations used in the Aerodynamic approach have names like "differential" and "quadratic", and it's really neat if you have a Cray handy to crunch your numbers! Too, if you want the true Cd for a given form, you have to test it. Extrapolation is fuzzy science. Anyway, that's my take on this FWIW, and I'm sure I need to read it a few more times. "Modern Exterior Ballistics", by Robert McCoy If you happen to buy this book, take a good look at the depicted/graphed Cd values as speed increases beyond the Mach 4 regions. It makes my madness seem a little less severe, my personal musings more worthy. [img]images/icons/wink.gif[/img] [/QUOTE]
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