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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Bullet stability
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<blockquote data-quote="Mikecr" data-source="post: 1336196" data-attributes="member: 1521"><p>Gyroscopic stability is easier to understand if you look at from a perspective of displacement per turn -to overcome that displacement.</p><p>That is, displacement(drag) is an overturning force, and each turn gyroscopically counters this.</p><p></p><p>This is how stability requirements are declared. For example: 8:1, is 8" of displacement per turn, and that displacement is tied to an atmospheric standard(usually same as BC is tied to).</p><p>Because velocity slows more than rotation rate, Sg usually goes up downrange as the bullet is displacing less and less overturning drag per each turn.</p><p></p><p>Normally, muzzle release is the biggest challenge to stability(muzzle pressure slapping the back of bullets where Sg is lowest). So if the bullet makes it 10yds without tumbling, it's usually good to go.</p><p>The next aerodynamic challenge is transonic. This is an issue, if dynamic stability is worse than gyroscopic stability can control. If I remember right, the 155smk is a known offender here.</p><p>Notice no bullet makers list or guarantee BC/Sg numbers at or below transonic. There are no public ballistic calculators that credit numbers at or below transonic as valid. This is because dynamic stability is an unknown.</p></blockquote><p></p>
[QUOTE="Mikecr, post: 1336196, member: 1521"] Gyroscopic stability is easier to understand if you look at from a perspective of displacement per turn -to overcome that displacement. That is, displacement(drag) is an overturning force, and each turn gyroscopically counters this. This is how stability requirements are declared. For example: 8:1, is 8" of displacement per turn, and that displacement is tied to an atmospheric standard(usually same as BC is tied to). Because velocity slows more than rotation rate, Sg usually goes up downrange as the bullet is displacing less and less overturning drag per each turn. Normally, muzzle release is the biggest challenge to stability(muzzle pressure slapping the back of bullets where Sg is lowest). So if the bullet makes it 10yds without tumbling, it's usually good to go. The next aerodynamic challenge is transonic. This is an issue, if dynamic stability is worse than gyroscopic stability can control. If I remember right, the 155smk is a known offender here. Notice no bullet makers list or guarantee BC/Sg numbers at or below transonic. There are no public ballistic calculators that credit numbers at or below transonic as valid. This is because dynamic stability is an unknown. [/QUOTE]
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Bullet stability
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