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Bullet Shock wave
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<blockquote data-quote="GeorgeS" data-source="post: 1522612" data-attributes="member: 107925"><p>For a given bullet, a certain amount of gyroscopic stability is required to stabilize its flight. When the bullet emerges from the barrel slowly, at a given twist rate, the spin is also slowed. For example, a 1:12 twist spins 1,000 r/s for every 1,000 f/s of velocity. If the bullet requires 1,500 r/s to stabilize at 1,000 f/s velocity, then it will be unstable out of a 1:12 twist; the twist would need to go to 1:8 (2/3rds of a foot) to accomplish that. </p><p></p><p>The force (spin angular momentum) required to resist destabilizing torques from aerodynamic forces and mass asymmetries decreases as the aerodynamic forces decrease at lower velocities, but in the transonic to subsonic transition realm, there's a big spike in these forces that causes the bullet to momentarily destabilize and yaw, then restabilize if the remaining spin is sufficient.</p><p></p><p>The situation is different down range for a bullet that has dropped to 1,000 f/s from an initial muzzle velocity of 2,000 f/s, because spin decays much more slowly than linear velocity. Initially at 2,000 r/s out of a 1:12 twist, the spin might have decayed to only 1,800 r/s downrange - more than enough to stabilize the bullet at that lower 1,000 f/s velocity. Subsonic it may be, but the destabilizing forces are still there.</p><p></p><p>I noticed the yaw as well - in the higher-speed pass, in fact. There's nothing like an empirical visualization to make what is happening clear.</p></blockquote><p></p>
[QUOTE="GeorgeS, post: 1522612, member: 107925"] For a given bullet, a certain amount of gyroscopic stability is required to stabilize its flight. When the bullet emerges from the barrel slowly, at a given twist rate, the spin is also slowed. For example, a 1:12 twist spins 1,000 r/s for every 1,000 f/s of velocity. If the bullet requires 1,500 r/s to stabilize at 1,000 f/s velocity, then it will be unstable out of a 1:12 twist; the twist would need to go to 1:8 (2/3rds of a foot) to accomplish that. The force (spin angular momentum) required to resist destabilizing torques from aerodynamic forces and mass asymmetries decreases as the aerodynamic forces decrease at lower velocities, but in the transonic to subsonic transition realm, there's a big spike in these forces that causes the bullet to momentarily destabilize and yaw, then restabilize if the remaining spin is sufficient. The situation is different down range for a bullet that has dropped to 1,000 f/s from an initial muzzle velocity of 2,000 f/s, because spin decays much more slowly than linear velocity. Initially at 2,000 r/s out of a 1:12 twist, the spin might have decayed to only 1,800 r/s downrange - more than enough to stabilize the bullet at that lower 1,000 f/s velocity. Subsonic it may be, but the destabilizing forces are still there. I noticed the yaw as well - in the higher-speed pass, in fact. There's nothing like an empirical visualization to make what is happening clear. [/QUOTE]
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