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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Let's argue about BC's
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<blockquote data-quote="Kevin Thomas" data-source="post: 475181" data-attributes="member: 15748"><p>Need a bit more information, but I can offer a few comments with what's here.</p><p> </p><p>1) you don't mention how much the BC changed. What I've seen many shooters freak out about really amounts to nothing more than the normal standard deviation seen within a string of shots when testing BC. Yes, they vary, sometimes substantially from shot to shot. </p><p> </p><p>2) were these bullets from the same box, or lot? Again, they can vary substantially from one production run to the next. Even more so, if we're talking about spitzers, given the trouble with achieving a constant blend and tip shape.</p><p> </p><p>3) Yes, atmosherics most definately DO play a role, and a fairly major one at that, when measuring BC. What you're dealing with is raw data, or an accurate BC for that time and place, which might be what you want. The manufacturers published BCs should all (and as far as I know, are) corrected back to ICAO or Std Metro corrections. This again allows a comparison for any given bullet against any other. In inputting altitude or pressure parameters in a ballistics program, you're in effect correcting this back to your conditions.</p><p> </p><p>4) Twist really makes very little difference here, until you reach the point at which a bullet verges on becoming unstable, then it makes a big time difference. I ran a series of tests with Bill McDonald on this that are in the 4th Edition Sierra manual, if I recall correctly. The plots are shown for several bullets in a range of five or six different twists for each caliber. They began with very high stability factors (SG) and proceeded on down to the point where they were nearing a SG of 1, or actually dipping below. The shots were plotted, and are there for all to see. I'll point out that the tests were run at 200 yards (which was what we had available) and that these figures may become more pronounced at geater distances.</p><p> </p><p>5) There's nothing wrong or antiquated about the G1 drag model. It was developed at precisely the same time (and by the same fellow) who gave us the G7. And the G5, G6, GL, etc., Mr. E.D. Lowrey of Olin-Winchester. The problem (as we see it today) is that the manufacturers standardized on this one drag model for everything, when it's not well suited to many. It IS the ideal drag model for some bullets, and dsitinctly not the right one for today's VLD style projectiles. The G7 is, likewise, not the ideal projectile for many sporting bullets, such as Spitzer flat based designs, round nose designs or a host of others. Bryan's piqued awareness of the fact that there's a better model for the VLDs than the G1, and that's what started the ball rolling. But to be precise, yes, there are other drag models out there and they'd be better used for the designs I mentioned. Getting complicated yet?</p><p> </p><p>6) velocity is an important factor, especially as range increases. Basically, we're seeing the actual performance diverge from the drag model's limitations in how they predict the downrange results. Using a VLD style bullet and the G1 drag model, the results will split pretty quickly and become more pronounced as the distance increases. Using the correct drag model, and those variations become much less, and ideally, stay spot on.</p><p> </p><p>7) Lastly, the use of multiple BCs is generally an attempt to account for this discrepancy, and make the results "fit" the trajectory we actually see on a shot fired in the field. It's a patch, and helps. The fix, is to use the correct drag model.</p></blockquote><p></p>
[QUOTE="Kevin Thomas, post: 475181, member: 15748"] Need a bit more information, but I can offer a few comments with what's here. 1) you don't mention how much the BC changed. What I've seen many shooters freak out about really amounts to nothing more than the normal standard deviation seen within a string of shots when testing BC. Yes, they vary, sometimes substantially from shot to shot. 2) were these bullets from the same box, or lot? Again, they can vary substantially from one production run to the next. Even more so, if we're talking about spitzers, given the trouble with achieving a constant blend and tip shape. 3) Yes, atmosherics most definately DO play a role, and a fairly major one at that, when measuring BC. What you're dealing with is raw data, or an accurate BC for that time and place, which might be what you want. The manufacturers published BCs should all (and as far as I know, are) corrected back to ICAO or Std Metro corrections. This again allows a comparison for any given bullet against any other. In inputting altitude or pressure parameters in a ballistics program, you're in effect correcting this back to your conditions. 4) Twist really makes very little difference here, until you reach the point at which a bullet verges on becoming unstable, then it makes a big time difference. I ran a series of tests with Bill McDonald on this that are in the 4th Edition Sierra manual, if I recall correctly. The plots are shown for several bullets in a range of five or six different twists for each caliber. They began with very high stability factors (SG) and proceeded on down to the point where they were nearing a SG of 1, or actually dipping below. The shots were plotted, and are there for all to see. I'll point out that the tests were run at 200 yards (which was what we had available) and that these figures may become more pronounced at geater distances. 5) There's nothing wrong or antiquated about the G1 drag model. It was developed at precisely the same time (and by the same fellow) who gave us the G7. And the G5, G6, GL, etc., Mr. E.D. Lowrey of Olin-Winchester. The problem (as we see it today) is that the manufacturers standardized on this one drag model for everything, when it's not well suited to many. It IS the ideal drag model for some bullets, and dsitinctly not the right one for today's VLD style projectiles. The G7 is, likewise, not the ideal projectile for many sporting bullets, such as Spitzer flat based designs, round nose designs or a host of others. Bryan's piqued awareness of the fact that there's a better model for the VLDs than the G1, and that's what started the ball rolling. But to be precise, yes, there are other drag models out there and they'd be better used for the designs I mentioned. Getting complicated yet? 6) velocity is an important factor, especially as range increases. Basically, we're seeing the actual performance diverge from the drag model's limitations in how they predict the downrange results. Using a VLD style bullet and the G1 drag model, the results will split pretty quickly and become more pronounced as the distance increases. Using the correct drag model, and those variations become much less, and ideally, stay spot on. 7) Lastly, the use of multiple BCs is generally an attempt to account for this discrepancy, and make the results "fit" the trajectory we actually see on a shot fired in the field. It's a patch, and helps. The fix, is to use the correct drag model. [/QUOTE]
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