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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Twist Rate???
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<blockquote data-quote="Kevin Thomas" data-source="post: 273131" data-attributes="member: 15748"><p>Hi Guys,</p><p></p><p>Actually, Sierra agrees completely with Bryan, or vice versa. I shot a series of TOF tests some years ago in both 22 and 30 calibers that took two bullets (.224" dia. 69 and .308" dia. 190 MKs) through a wide range of twists, ranging from excessively fast to completely inadeqate. The 190s were fired in; 1x8, 9, 10, 11, 12 and 14" twists, while the 69s were shot in 1x7, 8, 9, 10 and 12" twists, respectively. The bottom line was, there's very little difference in BC across a wide range of stability factors, until the bullet begins to reach a point of marginal stability. At that point, coning and precession begin to both reduce the measured BCs, and increase the standard deviations and extreme spreads of the sample population. I ran this with Bill McDonald, and he incorporated the results in the Exterior Ballistics chapter that he and Ted Almgren did in the Sierra 4th edition manual. The tests, complete with the scatter plots, velocities and resulting BCs for each twist are reproduced on pages 608-609 of that manual. </p><p></p><p>As far as negative results from overspinning a bullet, there really aren't any unless you have some jacket concentricity problems, or voids that result in off center CGs. Better to overspin than underspin in virtually all cases. </p><p></p><p>Kevin Thomas</p><p>Berger Bullets</p></blockquote><p></p>
[QUOTE="Kevin Thomas, post: 273131, member: 15748"] Hi Guys, Actually, Sierra agrees completely with Bryan, or vice versa. I shot a series of TOF tests some years ago in both 22 and 30 calibers that took two bullets (.224" dia. 69 and .308" dia. 190 MKs) through a wide range of twists, ranging from excessively fast to completely inadeqate. The 190s were fired in; 1x8, 9, 10, 11, 12 and 14" twists, while the 69s were shot in 1x7, 8, 9, 10 and 12" twists, respectively. The bottom line was, there's very little difference in BC across a wide range of stability factors, until the bullet begins to reach a point of marginal stability. At that point, coning and precession begin to both reduce the measured BCs, and increase the standard deviations and extreme spreads of the sample population. I ran this with Bill McDonald, and he incorporated the results in the Exterior Ballistics chapter that he and Ted Almgren did in the Sierra 4th edition manual. The tests, complete with the scatter plots, velocities and resulting BCs for each twist are reproduced on pages 608-609 of that manual. As far as negative results from overspinning a bullet, there really aren't any unless you have some jacket concentricity problems, or voids that result in off center CGs. Better to overspin than underspin in virtually all cases. Kevin Thomas Berger Bullets [/QUOTE]
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Twist Rate???
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