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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Help deciding on twist rate
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<blockquote data-quote="Mikecr" data-source="post: 1359163" data-attributes="member: 1521"><p>It would be better if you forget the notion of RPMs (it isn't). </p><p>Our bullets are inherently unstable in that their center of gravity is behind center of pressure. So they want to overturn, and high drag(affecting pressure amount/location) only serves to contribute more & more to that end.</p><p></p><p>Notice bullet stability requirements are not declared in RPMs. This is because turns per time does not really matter. </p><p>What does matter, and what bullet stability requirements are declared in, is displacement(or drag) per turn.</p><p>For example, a stability requirement may be 9 inches per 1 turn(9:1), which is always at some air density standard(while rarely declared this way). So ideal stability needs change with air density changes, BC(which is velocity dependent), or anything else affecting drag.</p><p>The turn is needed to gyroscopically overcome the overturning moments that displacement is contributing to.</p><p></p><p>If you would like an example of how RPMs fail with stability considerations, I can provide this. Or, you can go on faith that RPMs won't help.</p></blockquote><p></p>
[QUOTE="Mikecr, post: 1359163, member: 1521"] It would be better if you forget the notion of RPMs (it isn't). Our bullets are inherently unstable in that their center of gravity is behind center of pressure. So they want to overturn, and high drag(affecting pressure amount/location) only serves to contribute more & more to that end. Notice bullet stability requirements are not declared in RPMs. This is because turns per time does not really matter. What does matter, and what bullet stability requirements are declared in, is displacement(or drag) per turn. For example, a stability requirement may be 9 inches per 1 turn(9:1), which is always at some air density standard(while rarely declared this way). So ideal stability needs change with air density changes, BC(which is velocity dependent), or anything else affecting drag. The turn is needed to gyroscopically overcome the overturning moments that displacement is contributing to. If you would like an example of how RPMs fail with stability considerations, I can provide this. Or, you can go on faith that RPMs won't help. [/QUOTE]
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Help deciding on twist rate
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