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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
BALLISTIC COEFFICIENTS/Twist Rates
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<blockquote data-quote="LouBoyd" data-source="post: 417482" data-attributes="member: 9253"><p>Most ballistic computer programs are based on McCoy's McTrag computer program and do not account for any deviation in the spin axis and the direction of flight (ie yaw) of a bullet. The programs assume a bullet will fly nose into the wind. If a bullet is overspun it will fly "nose up as gravity tries to give it a roughly parabolic trajectory. I'm assuming you're measuring the effective BC from drop at the target and not by measuring the velocity at the target or time of flight. </p><p></p><p>You many not be measuring effective BC at all, just yaw effects. Yaw will actually reduce the BC if the bullet is not flying nose into the air, but a nose up bullet will have some lift at least until it precesses by 90 degrees. If' that's doesn't happen until over halfway to the target it will appear as a higher BC based on drop alone. You could sort out which is happening by measuring the velocity at the target or measuring the time of flight accurately. I don't know of a convenient method other than two chronographs or three chronograph screens and a digital oscilloscope with a good time base.</p><p></p><p>So which barrel gives the most accurate groups? That's much more important than which gives the least drop.</p><p>Also it would be good to know what elevation (altitude) and air temperature you're shooting in. Air density does matter in determining bullet stability.</p></blockquote><p></p>
[QUOTE="LouBoyd, post: 417482, member: 9253"] Most ballistic computer programs are based on McCoy's McTrag computer program and do not account for any deviation in the spin axis and the direction of flight (ie yaw) of a bullet. The programs assume a bullet will fly nose into the wind. If a bullet is overspun it will fly "nose up as gravity tries to give it a roughly parabolic trajectory. I'm assuming you're measuring the effective BC from drop at the target and not by measuring the velocity at the target or time of flight. You many not be measuring effective BC at all, just yaw effects. Yaw will actually reduce the BC if the bullet is not flying nose into the air, but a nose up bullet will have some lift at least until it precesses by 90 degrees. If' that's doesn't happen until over halfway to the target it will appear as a higher BC based on drop alone. You could sort out which is happening by measuring the velocity at the target or measuring the time of flight accurately. I don't know of a convenient method other than two chronographs or three chronograph screens and a digital oscilloscope with a good time base. So which barrel gives the most accurate groups? That's much more important than which gives the least drop. Also it would be good to know what elevation (altitude) and air temperature you're shooting in. Air density does matter in determining bullet stability. [/QUOTE]
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Rifles, Reloading, Optics, Equipment
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BALLISTIC COEFFICIENTS/Twist Rates
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