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Extreme Long Range Hunting & Shooting (ELR)
Twist rates for elr?
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<blockquote data-quote="Deleted member 71114" data-source="post: 1589340"><p>Here's a little explanation on the theories of Sg</p><p></p><p>Gyroscopic Stability</p><p>Various estimators for calculating the required rifling twist-rates for adequate gyroscopic stability Sg for rifle bullets have been used since the Greenhill Formula of the mid-1800's. We also now use Don Miller's formulation for VLD-type bullets and Bob McCoy's McGYRO calculations developed for artillery projectiles. A common feature of these estimators is that they all rely heavily upon the bullet length L in calibers as a type of slenderness ratio of the projectile. Some formulations also adjust for muzzle velocity, air density, and the average material density of the projectile.</p><p>Ideas are changing about what values of Sg constitute adequate and desired initial gyroscopic stability, especially in the flat-firing of rifles to extended ranges. Formerly, we considered an initial Sg of 1.2 to 1.4 to be adequate for best short-range rifle accuracy. Now, we realize the advantages of reduced aerodynamic drag if we provide our conventional jacketed, lead-cored rifle bullets with an initial Sg of at least 1.5, as recommended by Bob McCoy and Bryan Litz. Riflemen are now learning to launch the new monolithic copper-alloy ultra-low-drag (ULD) bullets with an initial Sg of 2.5 to 3.0 for best results in extreme long-range (ELR) shooting. This requires a rifling twist-rate of about 20 calibers per turn for monolithic ULD bullets of about 5.5 calibers in length L. The resulting bullet spin-rates at high muzzle speeds are not compatible with use of conventional jacketed lead-cored match bullets.</p><p></p><p></p><p>Cheers</p><p>osoh</p><p>JH</p></blockquote><p></p>
[QUOTE="Deleted member 71114, post: 1589340"] Here’s a little explanation on the theories of Sg Gyroscopic Stability Various estimators for calculating the required rifling twist-rates for adequate gyroscopic stability Sg for rifle bullets have been used since the Greenhill Formula of the mid-1800’s. We also now use Don Miller’s formulation for VLD-type bullets and Bob McCoy’s McGYRO calculations developed for artillery projectiles. A common feature of these estimators is that they all rely heavily upon the bullet length L in calibers as a type of slenderness ratio of the projectile. Some formulations also adjust for muzzle velocity, air density, and the average material density of the projectile. Ideas are changing about what values of Sg constitute adequate and desired initial gyroscopic stability, especially in the flat-firing of rifles to extended ranges. Formerly, we considered an initial Sg of 1.2 to 1.4 to be adequate for best short-range rifle accuracy. Now, we realize the advantages of reduced aerodynamic drag if we provide our conventional jacketed, lead-cored rifle bullets with an initial Sg of at least 1.5, as recommended by Bob McCoy and Bryan Litz. Riflemen are now learning to launch the new monolithic copper-alloy ultra-low-drag (ULD) bullets with an initial Sg of 2.5 to 3.0 for best results in extreme long-range (ELR) shooting. This requires a rifling twist-rate of about 20 calibers per turn for monolithic ULD bullets of about 5.5 calibers in length L. The resulting bullet spin-rates at high muzzle speeds are not compatible with use of conventional jacketed lead-cored match bullets. Cheers osoh JH [/QUOTE]
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