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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Berger Twist Rate Stability Calculator (need help)
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<blockquote data-quote="Hugnot" data-source="post: 2024658" data-attributes="member: 115658"><p>On the subject of stability and bullet failures.</p><p></p><p>RPM = MV * (12/twist) * 60, or RPM = MV * (720/twist)</p><p></p><p>An 87 VMax @ 3600 plus fps out of a 8 twist would have 324,000 RPM's & be spinning like crazy. Any major jacket defect would cause failure and contact with any object would result in a chaotic impact. Not good for deep controlled expansion on big animals like elks with common angling shots. Any variance of the bullets center of mass would be magnified by huge spin rates and accuracy would suffer. Shorter bullets don't need fast twist rates.</p><p></p><p>High performance has liabilities like short barrel life, heat, short brass life, and magnification of bullet defects. The advantages are flat trajectory and reduced wind deflection. Weatherby rifles and cartridges have been in use for a long time despite their drawbacks.</p><p></p><p>The Sg values are derived using the formula shown on my post that included the excel spread sheet. Sg data includes bullet diameter, weight, length, twist, velocity, temperature, & pressure. The major items in the equation are velocity, caliber, twist & length - these receive exponential treatment in the equation. The environmental stuff that varies with conditions is secondary.</p><p></p><p>Making a decision on terminal performance based on Sg values has problems. A 12 twist (common for .375's) .375, 300 grain, 2500 fps, 1.382 inch long bullet at 80*, used to kill big beasts, would have a Sg value of 3.11, as per the Miller Sg estimate equation. I have a .375- .338 and I use the 270 grain Speer SPBT bullet at about 2650, this has an Sg of 3.33; I like to shoot steel with this combo because I like to hear bullet impact sounds. Bullets having relatively high and equal sectional densities may have markedly different lengths thus the shorter bullet would have a higher Sg value but equal sectional density and velocity. The Miller equation does not include bullet shapes.</p><p></p><p>Some time ago I read about Berger bullets failing in flight. I have my own idea's why this happened, which is, the bullet jackets were work hardened before the ogive was formed. Upon forming the ogive, longitudinal stress fractures occurred along the ogive when the previously work hardened brass was subjected to additional stresses and the failure occurred at these cracks. I have seen these longitudinal cracks running along some bullet ogives. The longitudinal crack stuff has been observed to occur, at right angles, to the force applied, in many materials upon exceeding plastic limits. The failing Berger bullets were not subjected to huge velocities or excessive twist rates. The Miller Sg equation does not include jacket data or quality. Failure might have occurred with normal Sg values, like under Sg 2.0.</p><p></p><p>I have no plans for a .224 using 88 or 90 grain bullets. I shoot 75 .224 ELDM's at 3300 fps out of a 7.7 twist with no problems, SG 1.9, the Hornady 4DOF Sg calculation claims a 7.7 is too slow and the label on the box recommends a 7 twist.</p><p></p><p>If I had to shoot something weighing over 150 pounds with my small caliber .224's & 6mm's I would use one of those solid copper alloy bullets and would not care about any Sg values and would select a pointy bullet having good ballistic stuff.</p></blockquote><p></p>
[QUOTE="Hugnot, post: 2024658, member: 115658"] On the subject of stability and bullet failures. RPM = MV * (12/twist) * 60, or RPM = MV * (720/twist) An 87 VMax @ 3600 plus fps out of a 8 twist would have 324,000 RPM's & be spinning like crazy. Any major jacket defect would cause failure and contact with any object would result in a chaotic impact. Not good for deep controlled expansion on big animals like elks with common angling shots. Any variance of the bullets center of mass would be magnified by huge spin rates and accuracy would suffer. Shorter bullets don't need fast twist rates. High performance has liabilities like short barrel life, heat, short brass life, and magnification of bullet defects. The advantages are flat trajectory and reduced wind deflection. Weatherby rifles and cartridges have been in use for a long time despite their drawbacks. The Sg values are derived using the formula shown on my post that included the excel spread sheet. Sg data includes bullet diameter, weight, length, twist, velocity, temperature, & pressure. The major items in the equation are velocity, caliber, twist & length - these receive exponential treatment in the equation. The environmental stuff that varies with conditions is secondary. Making a decision on terminal performance based on Sg values has problems. A 12 twist (common for .375's) .375, 300 grain, 2500 fps, 1.382 inch long bullet at 80*, used to kill big beasts, would have a Sg value of 3.11, as per the Miller Sg estimate equation. I have a .375- .338 and I use the 270 grain Speer SPBT bullet at about 2650, this has an Sg of 3.33; I like to shoot steel with this combo because I like to hear bullet impact sounds. Bullets having relatively high and equal sectional densities may have markedly different lengths thus the shorter bullet would have a higher Sg value but equal sectional density and velocity. The Miller equation does not include bullet shapes. Some time ago I read about Berger bullets failing in flight. I have my own idea's why this happened, which is, the bullet jackets were work hardened before the ogive was formed. Upon forming the ogive, longitudinal stress fractures occurred along the ogive when the previously work hardened brass was subjected to additional stresses and the failure occurred at these cracks. I have seen these longitudinal cracks running along some bullet ogives. The longitudinal crack stuff has been observed to occur, at right angles, to the force applied, in many materials upon exceeding plastic limits. The failing Berger bullets were not subjected to huge velocities or excessive twist rates. The Miller Sg equation does not include jacket data or quality. Failure might have occurred with normal Sg values, like under Sg 2.0. I have no plans for a .224 using 88 or 90 grain bullets. I shoot 75 .224 ELDM's at 3300 fps out of a 7.7 twist with no problems, SG 1.9, the Hornady 4DOF Sg calculation claims a 7.7 is too slow and the label on the box recommends a 7 twist. If I had to shoot something weighing over 150 pounds with my small caliber .224's & 6mm's I would use one of those solid copper alloy bullets and would not care about any Sg values and would select a pointy bullet having good ballistic stuff. [/QUOTE]
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Berger Twist Rate Stability Calculator (need help)
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