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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Long bullets require fast twist barrels?
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<blockquote data-quote="Michael Courtney" data-source="post: 864667" data-attributes="member: 28191"><p>The attached graph shows the G1 ballistic coefficient plotted against the gyroscopic stability (Sg) computed with the Miller stability formula for the 53 grain solid copper Barnes TSX in .223 Rem. It is clear that the BC decreases as stability is lowered, but that the bullet is still stable at Sg close to 1.0. Subtracting 0.2 or 0.3 from the stability would imply that the bullet was still stable at Sg = 0.8 or Sg = 0.7 which is clearly nonsense. The bullet would easily tumble and not make it through the far chronograph (or have much more drag) if Sg were really 0.7 or 0.8. </p><p></p><p>Your suggestion to subtract 0.2 to 0.3 for mono-metal bullets and to go with a faster twist if in doubt may have some basis in your observations. But I expect that the propensity to add more "margin for error" arises from using high volume cartridges that give bullets a big kick in the backside as they leave the barrel, and perhaps a tendency to use bullets with higher dynamic instabilities. We've shot several CED monometal bullets at lower Sg and not seen them tumble. </p><p></p><p>The standard advice to keep predicted Sg above 1.4 or 1.5 is based on the understanding that most hobbyist shooters are not being careful with the details needed to accurately predict Sg. The margin for error is usually application error, as the formula itself has been shown to be accurate within 5%. The advice to get a faster twist barrel only applies when rebarrelling a rifle and costs about $1000 to apply. Many shooters apply the twist rule to see whether a given bullet is a good choice for an existing barrel and expected environment (shooting match or hunting trip). Careful attention to all the details can yield accurate results with the Miller twist rule, and one can be confident in the outcome with a predicted Sg as low as 1.3.</p></blockquote><p></p>
[QUOTE="Michael Courtney, post: 864667, member: 28191"] The attached graph shows the G1 ballistic coefficient plotted against the gyroscopic stability (Sg) computed with the Miller stability formula for the 53 grain solid copper Barnes TSX in .223 Rem. It is clear that the BC decreases as stability is lowered, but that the bullet is still stable at Sg close to 1.0. Subtracting 0.2 or 0.3 from the stability would imply that the bullet was still stable at Sg = 0.8 or Sg = 0.7 which is clearly nonsense. The bullet would easily tumble and not make it through the far chronograph (or have much more drag) if Sg were really 0.7 or 0.8. Your suggestion to subtract 0.2 to 0.3 for mono-metal bullets and to go with a faster twist if in doubt may have some basis in your observations. But I expect that the propensity to add more "margin for error" arises from using high volume cartridges that give bullets a big kick in the backside as they leave the barrel, and perhaps a tendency to use bullets with higher dynamic instabilities. We've shot several CED monometal bullets at lower Sg and not seen them tumble. The standard advice to keep predicted Sg above 1.4 or 1.5 is based on the understanding that most hobbyist shooters are not being careful with the details needed to accurately predict Sg. The margin for error is usually application error, as the formula itself has been shown to be accurate within 5%. The advice to get a faster twist barrel only applies when rebarrelling a rifle and costs about $1000 to apply. Many shooters apply the twist rule to see whether a given bullet is a good choice for an existing barrel and expected environment (shooting match or hunting trip). Careful attention to all the details can yield accurate results with the Miller twist rule, and one can be confident in the outcome with a predicted Sg as low as 1.3. [/QUOTE]
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Long bullets require fast twist barrels?
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