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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Ballistic Program Inconsistencies
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<blockquote data-quote="jeffjackson" data-source="post: 312248" data-attributes="member: 14728"><p>I got interested in this a few years ago while writing some of my own software.</p><p> </p><p>I have written software on my own using most of the available methods to compare and contrast; Numerical Analysis (like McCoy), Step-wise, Siacchi and Pejsa. This may get kind of geeky, but I think a lot of folks contribute differences to methods more than they should. Personally I found differences in how many different packages implemented and used drag curves. For instance, I originally had trouble getting the Siacchi I had implemented to match the McCoy (JBM) results. As I dug deeper (I have a background in math, computer science and physics) I realized that most of the published drag curve data that gets used with Siacchi and step-wise methods didn't match that used by most numerical analysis methods. To be more specific, if you look at McCoy's implementation he uses the CD vs. Mach table to express drag curve, Siacchi requires it be expressed in a certain set of curve formulas. When I went back and derived my piece wise curves to use with Siacchi from BRL published CD vs. Mach I found them to be different and started getting very close results with Siacchi well past 2000 yards. I also found that more packages gave more similar results for G1 than the other drag curves. That may be changing with more people looking at them than they did a few years ago.</p><p> </p><p>To adress the original posters question. First all, I found several packages to be plain wrong, but among those that are "correct" a lot of the differences you see that are small can be attributed to many things. One guy uses more precision for constants (such as gravity), one package may vary environment (like air density or gravity) over the trajectory where another guy keeps them constant. I think how many people convert pressure to station pressure may vary or be in accurate. Even how folks pick CD vs. Mach values can vary for a specific velocity, for example, one guy uses a simple linear interpolation and another guys uses something a little more sophisticated. I played around with several of these things and they make very little differences, a lot less than other variables such as BC or environment can be measured, but they do result in many different packages giving slightly different results.</p><p> </p><p>If you really want to see packages give different results start changing away from standard environmental conditions, I found packages to start varying wildly, especially with temperature.</p></blockquote><p></p>
[QUOTE="jeffjackson, post: 312248, member: 14728"] I got interested in this a few years ago while writing some of my own software. I have written software on my own using most of the available methods to compare and contrast; Numerical Analysis (like McCoy), Step-wise, Siacchi and Pejsa. This may get kind of geeky, but I think a lot of folks contribute differences to methods more than they should. Personally I found differences in how many different packages implemented and used drag curves. For instance, I originally had trouble getting the Siacchi I had implemented to match the McCoy (JBM) results. As I dug deeper (I have a background in math, computer science and physics) I realized that most of the published drag curve data that gets used with Siacchi and step-wise methods didn't match that used by most numerical analysis methods. To be more specific, if you look at McCoy's implementation he uses the CD vs. Mach table to express drag curve, Siacchi requires it be expressed in a certain set of curve formulas. When I went back and derived my piece wise curves to use with Siacchi from BRL published CD vs. Mach I found them to be different and started getting very close results with Siacchi well past 2000 yards. I also found that more packages gave more similar results for G1 than the other drag curves. That may be changing with more people looking at them than they did a few years ago. To adress the original posters question. First all, I found several packages to be plain wrong, but among those that are "correct" a lot of the differences you see that are small can be attributed to many things. One guy uses more precision for constants (such as gravity), one package may vary environment (like air density or gravity) over the trajectory where another guy keeps them constant. I think how many people convert pressure to station pressure may vary or be in accurate. Even how folks pick CD vs. Mach values can vary for a specific velocity, for example, one guy uses a simple linear interpolation and another guys uses something a little more sophisticated. I played around with several of these things and they make very little differences, a lot less than other variables such as BC or environment can be measured, but they do result in many different packages giving slightly different results. If you really want to see packages give different results start changing away from standard environmental conditions, I found packages to start varying wildly, especially with temperature. [/QUOTE]
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