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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Stability: Fine Points to be Aware Of
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<blockquote data-quote="nralifer" data-source="post: 2633665" data-attributes="member: 94556"><p>Beg to differ. Early pn we made a 320 gr .338 that from a 1:9 twist at 100 yds at 60 degrees grouped one hole for 5 shots. Later in the winter at 5 degrees we shot the same bullet to 500 yds and every hit was sideways. Clearly the air was denser and there was a progressive tumbling that developed as the bullet traveled down range. It is the spin of the bullet that gives it its gyroscopic stability and long bullets have to be spun faster to maintain stability. Discussed the issue with some ballistisian from Hornady who said that was impossible, yet we obsrved it plain as day. If you take the JBM Stability Calculator and progressively increase the atmospheric pressure in the calculator you will see that the stability factor decreases. Do the opposite and it increases. Inadequately stabilized bullets will definitely tumble in gel if marginally stable in air. Why do you suppose subsonic bullets need tighter twists to stabilize? Gyroscopes become more stable the faster you spin them, and as they lose angular momentum they become more unstable. The basic gyroscopic principle is that a spining gyroscope resists torquing forces perpendicular to the spin axis. Bullets shot from rifled barrels have spin axes collinear with the direction of flight.</p></blockquote><p></p>
[QUOTE="nralifer, post: 2633665, member: 94556"] Beg to differ. Early pn we made a 320 gr .338 that from a 1:9 twist at 100 yds at 60 degrees grouped one hole for 5 shots. Later in the winter at 5 degrees we shot the same bullet to 500 yds and every hit was sideways. Clearly the air was denser and there was a progressive tumbling that developed as the bullet traveled down range. It is the spin of the bullet that gives it its gyroscopic stability and long bullets have to be spun faster to maintain stability. Discussed the issue with some ballistisian from Hornady who said that was impossible, yet we obsrved it plain as day. If you take the JBM Stability Calculator and progressively increase the atmospheric pressure in the calculator you will see that the stability factor decreases. Do the opposite and it increases. Inadequately stabilized bullets will definitely tumble in gel if marginally stable in air. Why do you suppose subsonic bullets need tighter twists to stabilize? Gyroscopes become more stable the faster you spin them, and as they lose angular momentum they become more unstable. The basic gyroscopic principle is that a spining gyroscope resists torquing forces perpendicular to the spin axis. Bullets shot from rifled barrels have spin axes collinear with the direction of flight. [/QUOTE]
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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Stability: Fine Points to be Aware Of
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