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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
18 inch 1:14 22-250
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<blockquote data-quote="Mikecr" data-source="post: 2033041" data-attributes="member: 1521"><p>The bullet has to turn pretty much exactly with the rifling. A given twist rate is imparted with 40" rifle barrels just the same as with 4" handgun barrels.</p><p></p><p>You might hold a notion(as many do) that stability is tied to RPMs. But there is no TIME in stability calcs. No velocity in feet per <em>SECOND</em>, nor revolutions per <em>MINUTE</em>.</p><p>Stability is a matter of a given bullet's relative displacement per turn(like 14"per turn). The displacement is relative because that 14" represents a quantity of drag, which is overturning to the bullet. Drag is affected by velocity, and this is where folks get the idea that velocity is significant to stability. But it really isn't. </p><p>Even the drag hit at transonic is not as significant to stability as bullet release from muzzle. That point, muzzle release, is the biggest challenge to overcome, and why bullets tumbling do so immediately.</p><p>As a bullet travels down range, it's turn rate drops fairly slowly, while it's displacement per each turn reduces way more. So as velocity decreases all the way, gyroscopic stability goes up all the way. With this, by the time a bullet has slowed to transonic, it would take more than that drag jump to tumble a bullet. So all is fine there.</p><p>Higher air density(like at sea level on a cold day) makes 14" of displacement way different than at lower air densities(summer in the mountains).</p><p></p><p>I just referred to bullet turn rate, and RPM thinkers would jump on that. </p><p>It's wrong thinking to do so though. </p><p>Stability is not affected by turn rate, but by displacement per turn, regardless of turn rate.</p><p>It's important to latch onto this reality, because it's far more important to consider the bullet and air density for needed twist rates, than velocities.</p><p></p><p>It's very rare when velocity is significant to stability. And if there, a bump in velocity just prevents tumbling, you're a long way from better solution (correct bullet or twist).</p></blockquote><p></p>
[QUOTE="Mikecr, post: 2033041, member: 1521"] The bullet has to turn pretty much exactly with the rifling. A given twist rate is imparted with 40" rifle barrels just the same as with 4" handgun barrels. You might hold a notion(as many do) that stability is tied to RPMs. But there is no TIME in stability calcs. No velocity in feet per [I]SECOND[/I], nor revolutions per [I]MINUTE[/I]. Stability is a matter of a given bullet's relative displacement per turn(like 14"per turn). The displacement is relative because that 14" represents a quantity of drag, which is overturning to the bullet. Drag is affected by velocity, and this is where folks get the idea that velocity is significant to stability. But it really isn't. Even the drag hit at transonic is not as significant to stability as bullet release from muzzle. That point, muzzle release, is the biggest challenge to overcome, and why bullets tumbling do so immediately. As a bullet travels down range, it's turn rate drops fairly slowly, while it's displacement per each turn reduces way more. So as velocity decreases all the way, gyroscopic stability goes up all the way. With this, by the time a bullet has slowed to transonic, it would take more than that drag jump to tumble a bullet. So all is fine there. Higher air density(like at sea level on a cold day) makes 14" of displacement way different than at lower air densities(summer in the mountains). I just referred to bullet turn rate, and RPM thinkers would jump on that. It's wrong thinking to do so though. Stability is not affected by turn rate, but by displacement per turn, regardless of turn rate. It's important to latch onto this reality, because it's far more important to consider the bullet and air density for needed twist rates, than velocities. It's very rare when velocity is significant to stability. And if there, a bump in velocity just prevents tumbling, you're a long way from better solution (correct bullet or twist). [/QUOTE]
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18 inch 1:14 22-250
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