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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Twist rate vs velocity
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<blockquote data-quote="Mikecr" data-source="post: 283654" data-attributes="member: 1521"><p>I don't know if Katbird is addressing your question, but his 'twist understanding dump' should really be corrected/enhanced..</p><p></p><p></p><p></p><p>My goal is to FULLY stabilize bullets, as soon as possible.</p><p></p><p></p><p></p><p>This is only a general rule, in a very limited sense. I'm sure many have screwed themselves with this notion.</p><p>RPMs do go up with velocity, and stability usually goes up with velocity(a very tiny amount), but not necessarily.</p><p>It depends on where the velocity puts you on that particular bullet's drag curve.</p><p></p><p>Reason being, stability is tied to displacement per turn. It's the upsetting quantity for each turn to overcome. With this, bullet twist requirements are expressed as such (like 8"per 1 turn) based on 8" of standard atmosphere displaced, and the drag coefficient of that bullet, at chosen velocity, per 1 turn. </p><p></p><p>There are other physical attributes of bullets affecting stability, but TIME never comes into play. </p><p>Stability doesn't really follow turns per time(RPMs). If it did, bullet stability requirements would be expressed in RPMs,,, and none are. </p><p>They never will ever be, because a time based definition of one condition would fail many tests elsewhere.</p><p>-For instance, a bullet barely stable at 80deg, can whack paper sideways at 40deg. Same velocity, same RPMs, but different displacement(denser atmosphere).</p><p>-In contrast, an unstable bullet at 1,500fps MV can punch round holes at 2,500fps. A huge improvement, but the bullet is far from fully stable, and would likely perform poorly. It can be the 40deg mishap above in waiting.</p><p>So velocity helped, yet it would take an impossible 15,000fps MV to fully stabilize this bullet, with this twist and condition.</p><p>So why such a gigantic change in RPMs to improve stability(from 171Krpm to 1029Krpm)? It's because RPMs never mattered..</p><p>I used 10.5:1 twist for this test and whether 1,500fps or 15,000 the displacement was still 10.5" per turn. The difference, all along, came down to atmosphere and the bullet's drag curve. </p><p>I could shoot it FULLY stable, at 2500fps, with use at 90deg and 6000' above sea level. Or I could choose a 9.5tw barrel from the beginning for use at 90deg at sea level. Or I could choose a 9tw barrel and always be fully stable.</p><p></p><p>I'm no expert, so you might run tests for yourself at JBMs.</p></blockquote><p></p>
[QUOTE="Mikecr, post: 283654, member: 1521"] I don't know if Katbird is addressing your question, but his 'twist understanding dump' should really be corrected/enhanced.. My goal is to FULLY stabilize bullets, as soon as possible. This is only a general rule, in a very limited sense. I'm sure many have screwed themselves with this notion. RPMs do go up with velocity, and stability usually goes up with velocity(a very tiny amount), but not necessarily. It depends on where the velocity puts you on that particular bullet's drag curve. Reason being, stability is tied to displacement per turn. It's the upsetting quantity for each turn to overcome. With this, bullet twist requirements are expressed as such (like 8"per 1 turn) based on 8" of standard atmosphere displaced, and the drag coefficient of that bullet, at chosen velocity, per 1 turn. There are other physical attributes of bullets affecting stability, but TIME never comes into play. Stability doesn't really follow turns per time(RPMs). If it did, bullet stability requirements would be expressed in RPMs,,, and none are. They never will ever be, because a time based definition of one condition would fail many tests elsewhere. -For instance, a bullet barely stable at 80deg, can whack paper sideways at 40deg. Same velocity, same RPMs, but different displacement(denser atmosphere). -In contrast, an unstable bullet at 1,500fps MV can punch round holes at 2,500fps. A huge improvement, but the bullet is far from fully stable, and would likely perform poorly. It can be the 40deg mishap above in waiting. So velocity helped, yet it would take an impossible 15,000fps MV to fully stabilize this bullet, with this twist and condition. So why such a gigantic change in RPMs to improve stability(from 171Krpm to 1029Krpm)? It's because RPMs never mattered.. I used 10.5:1 twist for this test and whether 1,500fps or 15,000 the displacement was still 10.5" per turn. The difference, all along, came down to atmosphere and the bullet's drag curve. I could shoot it FULLY stable, at 2500fps, with use at 90deg and 6000' above sea level. Or I could choose a 9.5tw barrel from the beginning for use at 90deg at sea level. Or I could choose a 9tw barrel and always be fully stable. I'm no expert, so you might run tests for yourself at JBMs. [/QUOTE]
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