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Bullets Spinning Apart
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<blockquote data-quote="Randy Tidwell" data-source="post: 1687344" data-attributes="member: 109136"><p>I shoot allot of fast twist barrels. At one point I was experiencing some 105 VLD bullets coming apart in flight. I was running them at about 3250 fps from a version of 6mmAI</p><p></p><p>I called Berger about it and was told that they should not be driven over 3150 from an 8 twist barrel because they spin apart.</p><p></p><p>So, I decided to calculate just what the RPM of a bullet would be from an 8 twist barrel at 3150 fps. I wrote a quick little spread sheet to make this calculation.</p><p></p><p>283,500 RPM, holy crap, this can't be right? Think about the centrifugal force on the thin copper jacket at that RPM, no wonder they were coming apart.</p><p></p><p>I called my son that has a Masters in Math to confirm my calculation. I only gave him the twist rate and velocity. He confirmed I was correct.</p><p></p><p>The formula to calculate RPM is: </p><p>Velocity * 60 *12 / Twist Rate</p><p></p><p>3150 fps * 60 = 189,000 feet Per Minute</p><p>189,00 fpm * 12 = 2,268,000 Inches per minute</p><p>2,268,000 inches/min / 8" Twist Rate = 283,500</p><p></p><p>I tried to attach a spread sheet that you can make RPM calculations for different barrel twist rates and velocity, but the extension is not acceptable. </p><p></p><p>If anyone wants a copy of the spread sheet, send me your email address, I will be happy to send it to you.</p></blockquote><p></p>
[QUOTE="Randy Tidwell, post: 1687344, member: 109136"] I shoot allot of fast twist barrels. At one point I was experiencing some 105 VLD bullets coming apart in flight. I was running them at about 3250 fps from a version of 6mmAI I called Berger about it and was told that they should not be driven over 3150 from an 8 twist barrel because they spin apart. So, I decided to calculate just what the RPM of a bullet would be from an 8 twist barrel at 3150 fps. I wrote a quick little spread sheet to make this calculation. 283,500 RPM, holy crap, this can't be right? Think about the centrifugal force on the thin copper jacket at that RPM, no wonder they were coming apart. I called my son that has a Masters in Math to confirm my calculation. I only gave him the twist rate and velocity. He confirmed I was correct. The formula to calculate RPM is: Velocity * 60 *12 / Twist Rate 3150 fps * 60 = 189,000 feet Per Minute 189,00 fpm * 12 = 2,268,000 Inches per minute 2,268,000 inches/min / 8" Twist Rate = 283,500 I tried to attach a spread sheet that you can make RPM calculations for different barrel twist rates and velocity, but the extension is not acceptable. If anyone wants a copy of the spread sheet, send me your email address, I will be happy to send it to you. [/QUOTE]
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