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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: 864539" data-attributes="member: 28191"><p>The density is NOT a required input for the Miller twist rule. The calc on Berger's site does not depend on the bullet density. It depends on the length, weight, and caliber to be accurately entered, as well as the twist rate and atmospheric conditions. We've used the twist rule to accurately predict stability of several Barnes X bullets. </p><p></p><p>One always needs to ask what the evidence was that the bullet was not stable. Less than desired accuracy is NOT evidence of instability. High speed video of tumbling bullets or keyholes in paper targets is convincing evidence of instability. Inaccuracy can have many other causal factors. Careful determinations of BC vs. muzzle velocity can indicate marginal stability. </p><p></p><p>The numbers you cite above would require the bullet to be no longer than 1.5". What's the actual bullet length for the CEB 140 grain 6.5? Also, be mindful that a cartridge that shoots a 6.5 140 grain bullet at 3200 + fps is going to have an abnormally high pressure when the bullet leaves the barrel. Such a large pressure on the base as the bullet leaves the barrel is going to be more likely to induce tumbling than normal cartridges. The twist rule predicts stability for a bullet in free flight, with no additional forces than air drag. The twist rule does not predict stability when subjected to significant additional forces. </p><p></p><p>Finally, be mindful that the predicted stability depends strongly on an accurate twist rate. If the actual twist rate were 1 in 8.6" rather than 1 in 8.4", the Sg would be reduced from the predicted 1.31 to 1.25. A 1 in 9.0" twist rate would produce an Sg of 1.14. I would recommend that twist rates actually be measured. Trusting gunsmiths or manufacturers can lead to inaccurate results.</p></blockquote><p></p>
[QUOTE="Michael Courtney, post: 864539, member: 28191"] The density is NOT a required input for the Miller twist rule. The calc on Berger's site does not depend on the bullet density. It depends on the length, weight, and caliber to be accurately entered, as well as the twist rate and atmospheric conditions. We've used the twist rule to accurately predict stability of several Barnes X bullets. One always needs to ask what the evidence was that the bullet was not stable. Less than desired accuracy is NOT evidence of instability. High speed video of tumbling bullets or keyholes in paper targets is convincing evidence of instability. Inaccuracy can have many other causal factors. Careful determinations of BC vs. muzzle velocity can indicate marginal stability. The numbers you cite above would require the bullet to be no longer than 1.5". What's the actual bullet length for the CEB 140 grain 6.5? Also, be mindful that a cartridge that shoots a 6.5 140 grain bullet at 3200 + fps is going to have an abnormally high pressure when the bullet leaves the barrel. Such a large pressure on the base as the bullet leaves the barrel is going to be more likely to induce tumbling than normal cartridges. The twist rule predicts stability for a bullet in free flight, with no additional forces than air drag. The twist rule does not predict stability when subjected to significant additional forces. Finally, be mindful that the predicted stability depends strongly on an accurate twist rate. If the actual twist rate were 1 in 8.6" rather than 1 in 8.4", the Sg would be reduced from the predicted 1.31 to 1.25. A 1 in 9.0" twist rate would produce an Sg of 1.14. I would recommend that twist rates actually be measured. Trusting gunsmiths or manufacturers can lead to inaccurate results. [/QUOTE]
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Long bullets require fast twist barrels?
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