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Rifles, Reloading, Optics, Equipment
Reloading
Barrel length and twist rate
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<blockquote data-quote="Mikecr" data-source="post: 2788084" data-attributes="member: 1521"><p>Let me expand on this with something fun (external ballistics) to think about.</p><p>We initially have 9" displacement per turn here, and let's say that this is providing a gyroscopic stability (Sg) of 1.2.</p><p>That's at the muzzle, on bullet release, and it's marginal. The bullet rarely releases perfectly clean, and it takes a few feet for gyroscopic forces to bring the bullet fully point forward. That first few feet of the muzzle represents the biggest gyroscopic stability challenge.</p><p>If the bullet does not tumble immediately, then it is well on it's way, with Sg climbing from there, all the way to transonic.</p><p></p><p>The Sg climbs because the effective displacement per turn is changing.</p><p>It's getting tighter and tighter, as the bullet velocity slows faster than it's turn rate.</p><p>An example Sierra 111 DTAC, ICAO air density, 3000fps, 9:1 twist</p><p></p><p>Muzz Sg = 1.16, Vel 3000, Eff Twist 9.0 [240krpm]</p><p>50yd Sg = 1.22, Vel 2917, ETW 8.8 [239krpm]</p><p>100yd Sg = 1.27, Vel 2834, ETW 8.6 [237krpm]</p><p>200yd Sg = 1.38, Vel 2674, ETW 8.2 [234krpm]</p><p>278yd Sg = 1.50, Vel 2553, ETW 7.9 [232krpm] Fully Stable (sleep)</p><p>500yd Sg = 1.88, Vel 2226, ETW 7.1 [226krpm]</p><p>1Kyd Sg = 3.34, Vel 1582, ETW 5.3 [215krpm]</p><p></p><p>I threw RPMs in just to show that RPMs are going down while Sg is going up.</p><p>Let's also look at hypothetical muzzle velocity affect to this bullet's Sg from 9tw:</p><p></p><p>3,000fps Sg = 1.16</p><p>3,500fps Sg = 1.19</p><p>4,000fps Sg = 1.21</p><p>10,000fps Sg = 1.34</p><p>17,513fps Sg = 1.50 fully stable, and on it's way to Proxima Centauri at 1.4 million RPMs.</p><p></p><p> Ok, that's fun right there. I don't care who you are</p></blockquote><p></p>
[QUOTE="Mikecr, post: 2788084, member: 1521"] Let me expand on this with something fun (external ballistics) to think about. We initially have 9" displacement per turn here, and let's say that this is providing a gyroscopic stability (Sg) of 1.2. That's at the muzzle, on bullet release, and it's marginal. The bullet rarely releases perfectly clean, and it takes a few feet for gyroscopic forces to bring the bullet fully point forward. That first few feet of the muzzle represents the biggest gyroscopic stability challenge. If the bullet does not tumble immediately, then it is well on it's way, with Sg climbing from there, all the way to transonic. The Sg climbs because the effective displacement per turn is changing. It's getting tighter and tighter, as the bullet velocity slows faster than it's turn rate. An example Sierra 111 DTAC, ICAO air density, 3000fps, 9:1 twist Muzz Sg = 1.16, Vel 3000, Eff Twist 9.0 [240krpm] 50yd Sg = 1.22, Vel 2917, ETW 8.8 [239krpm] 100yd Sg = 1.27, Vel 2834, ETW 8.6 [237krpm] 200yd Sg = 1.38, Vel 2674, ETW 8.2 [234krpm] 278yd Sg = 1.50, Vel 2553, ETW 7.9 [232krpm] Fully Stable (sleep) 500yd Sg = 1.88, Vel 2226, ETW 7.1 [226krpm] 1Kyd Sg = 3.34, Vel 1582, ETW 5.3 [215krpm] I threw RPMs in just to show that RPMs are going down while Sg is going up. Let's also look at hypothetical muzzle velocity affect to this bullet's Sg from 9tw: 3,000fps Sg = 1.16 3,500fps Sg = 1.19 4,000fps Sg = 1.21 10,000fps Sg = 1.34 17,513fps Sg = 1.50 fully stable, and on it's way to Proxima Centauri at 1.4 million RPMs. Ok, that's fun right there. I don't care who you are [/QUOTE]
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