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Straight up, what a fantasitic effort by the authors and researchers, especially considering the equipment employed.
First question is with regard to the calibration process. I didn't identify at which distance the calibration was done. If calibration was done at all three distances (10, 160 & 320 feet), was the difference in readings between the three chronographs still within error limits? I say this, given the tendancy of optical chronographs to be light sensative; and the light might have changed with distance(position), let alone time of day and cloud conditions.
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The calibration was done with the three chronographs at 10, 12, and 14 feet from the muzzle. The LED skyscreens greatly reduce the influence of changing light conditions. We've calibrated the chronographs many different times (different days, different light conditions, etc.) and we've never had them fail to meet the 0.3% specification with the LED skyscreens, and the accuracy outside is not distinctly worse than the accuracy inside.
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Do you believe the spin, pitch & yaw paths to be truely circular or elipitcal as found more often in nature? If the path is more eliptical and prone to changing with velocity and distance might this impact the measured drag coeffcient each time. I'm assuming that the pitch and yaw act in combination to increase drag, but the combination would have a different effect if the pitch was at the major axis of the elipse and the yaw was only at the minor axis and vice-versa.
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The shape of the pitch and yaw is best described in the above video which is referenced in the paper. The video describes it much better than simple verbal descriptions like circular or elliptical. The theory really is very well worked out by Bryan Litz (see also
Epicyclic Swerve ) based on the earlier work of Braun and McCoy. The only thing our new experimental technique reveals is how big the effect is for a specific bullet and rifle and how quickly the pitch and yaw are damped in flight for a specific bullet and rifle. The theory can describe the subsequent motion for a given set of initial conditions, but the magnitude of the effect for a given rifle and bullet depend on the initial conditions.
It is not completely clear if the shot-to-shot variations in drag are due to shot-to-shot variations in tip off rate, manufacturing variations in different bullets, or other experimental contributions to the uncertainty. We've managed to achieve Cd measurements with accuracy in the 1-2% range over the 100 yard interval and in the 3-4% range over the two 50 yard intervals with less than $1k of equipment. $100k of equipment could reduce the experimental error, but it still would not tell you whether you were seeing shot-to-shot variations in the tip off rate or in the Cd of different bullets. We did intentionally pick a bullet where we had observed small shot-to-shot drag variations in the past.
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Finally, were any chronographs harmed during the making of this report?
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You may be remembering the stability papers co-authored with Don Miller where I shot the downrange chronograph. Since that event (Summer 2011), we've been much more careful. Our main upgrade in chronograph safety is having Elya or Amy behind the trigger, while I've been demoted to data recorder when we take data. Elya was the shooter for this experiment, so the chronographs were safe.
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