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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
OVERSPIN ??? twist and bullet spin
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<blockquote data-quote="Gerard Schultz" data-source="post: 907878" data-attributes="member: 51"><p>My take on twist is:</p><p></p><p>1. Jacketed bullets are more affected by overspin than monometal bullets. As mentioned, spin a jacketed bullet fast enough and centripetal force will pull it to pieces. Also, as mentioned, any anomaly in jacket thickness or inclusions (dirt, air) in the core will be enhanced by a tighter twist. Monometal bullets do not have these problems as long as bullet weight is constant within less than 0.5%.</p><p></p><p>2. Stability across the trajectory falls into three major categories: Gyroscopic stability (Sg), dynamic stability (Sd) and tractability. The requirement of punching paper and steel, differ from the requirement of terminal performance in tissue.</p><p></p><p>3. Stable flight will generally happen when the gyroscopic stability (Sg) of a bullet is above 1.1/1.2. It is accepted that stable flight is a given with the Sg above one but that is sailing very close to the wind. So, for target and tactical shooting, stable flight is ok. For hunting, where there is the additional requirement of terminal performance, just having stable flight is not ok. The Sg at launch must be higher and one bullet cannot do all.</p><p></p><p>4. If a bullet has gyroscopic stability at launch, it will have gyroscopic stability across the entire trajectory. A bullet may destabilize because it starts yawing as a result of becoming dynamically unstable due too high a launch speed. That is a design parameter and it is important to pick a bullet that is intended to be launched at more than what your rifle is capable of, to be safe.</p><p></p><p>5. Tractability only becomes a factor with typical .284"/.300" caliber when distance is over 500/600.</p><p></p><p>6. Overspin (high Sg) is especially useful for short distance shooting with monometal bullets such as for bush and dangerous game.</p><p></p><p>7. The original question: An 8" twist in a .300" caliber would be very good for hunting to 600/700 with long bullets in mono around 180gr and in jacketed lead 210 and up. For further than 700, longer monos must be looked at.</p></blockquote><p></p>
[QUOTE="Gerard Schultz, post: 907878, member: 51"] My take on twist is: 1. Jacketed bullets are more affected by overspin than monometal bullets. As mentioned, spin a jacketed bullet fast enough and centripetal force will pull it to pieces. Also, as mentioned, any anomaly in jacket thickness or inclusions (dirt, air) in the core will be enhanced by a tighter twist. Monometal bullets do not have these problems as long as bullet weight is constant within less than 0.5%. 2. Stability across the trajectory falls into three major categories: Gyroscopic stability (Sg), dynamic stability (Sd) and tractability. The requirement of punching paper and steel, differ from the requirement of terminal performance in tissue. 3. Stable flight will generally happen when the gyroscopic stability (Sg) of a bullet is above 1.1/1.2. It is accepted that stable flight is a given with the Sg above one but that is sailing very close to the wind. So, for target and tactical shooting, stable flight is ok. For hunting, where there is the additional requirement of terminal performance, just having stable flight is not ok. The Sg at launch must be higher and one bullet cannot do all. 4. If a bullet has gyroscopic stability at launch, it will have gyroscopic stability across the entire trajectory. A bullet may destabilize because it starts yawing as a result of becoming dynamically unstable due too high a launch speed. That is a design parameter and it is important to pick a bullet that is intended to be launched at more than what your rifle is capable of, to be safe. 5. Tractability only becomes a factor with typical .284"/.300" caliber when distance is over 500/600. 6. Overspin (high Sg) is especially useful for short distance shooting with monometal bullets such as for bush and dangerous game. 7. The original question: An 8" twist in a .300" caliber would be very good for hunting to 600/700 with long bullets in mono around 180gr and in jacketed lead 210 and up. For further than 700, longer monos must be looked at. [/QUOTE]
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OVERSPIN ??? twist and bullet spin
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