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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
.220 Swift - Non Lead - 1:14 twist
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<blockquote data-quote="Mikecr" data-source="post: 2158515" data-attributes="member: 1521"><p>Scope-eye, you're totally wrong. An RPM calc is not a stability calc.</p><p>There is no 'time' in stability requirements, but displacement per revolution(each).</p><p>And no credible bullet maker would ever declare stability requirements in RPMs. Have you ever noticed that?</p><p></p><p>Example: a typical 55gr 22cal bullet under standard ICAO conditions(SL at 59degF):</p><p> <u>in 14tw</u></p><p>Sg = 1.06 at 2kfps, kRPM 103</p><p>Sg = 1.11 at 3kfps, kRPM 154</p><p>Sg = 1.14 at 4kfps, kRPM 206</p><p>Sg = 1.24 at <span style="color: rgb(184, 49, 47)">10kfps</span>, kRPM 514 (still marginal)</p><p></p><p> <u>in 12tw</u></p><p>Sg = 1.44 at 2kfps, kRPM 120</p><p>Sg = 1.52 at 3kfps, kRPM 180</p><p>Sg = 1.56 at 4kfps, kRPM 240</p><p>Sg = 1.69 at <span style="color: rgb(184, 49, 47)">10kfps</span>, kRPM 600</p><p></p><p>You can see from this that there is no direct correlation between RPM and stability, and that no realistic amount of RPM's could provide full stability in 14tw with that bullet.</p><p>Well, unless you consider 27,000fps / 1,337,000 RPMs realistic....</p><p></p><p>What DOES make a difference is the relative displacement:</p><p>Same bullet, in 14tw, 2kfps, but 100degF and 7800' elevation, Sg = 1.50</p></blockquote><p></p>
[QUOTE="Mikecr, post: 2158515, member: 1521"] Scope-eye, you're totally wrong. An RPM calc is not a stability calc. There is no 'time' in stability requirements, but displacement per revolution(each). And no credible bullet maker would ever declare stability requirements in RPMs. Have you ever noticed that? Example: a typical 55gr 22cal bullet under standard ICAO conditions(SL at 59degF): [U]in 14tw[/U] Sg = 1.06 at 2kfps, kRPM 103 Sg = 1.11 at 3kfps, kRPM 154 Sg = 1.14 at 4kfps, kRPM 206 Sg = 1.24 at [COLOR=rgb(184, 49, 47)]10kfps[/COLOR], kRPM 514 (still marginal) [U]in 12tw[/U] Sg = 1.44 at 2kfps, kRPM 120 Sg = 1.52 at 3kfps, kRPM 180 Sg = 1.56 at 4kfps, kRPM 240 Sg = 1.69 at [COLOR=rgb(184, 49, 47)]10kfps[/COLOR], kRPM 600 You can see from this that there is no direct correlation between RPM and stability, and that no realistic amount of RPM's could provide full stability in 14tw with that bullet. Well, unless you consider 27,000fps / 1,337,000 RPMs realistic.... What DOES make a difference is the relative displacement: Same bullet, in 14tw, 2kfps, but 100degF and 7800' elevation, Sg = 1.50 [/QUOTE]
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.220 Swift - Non Lead - 1:14 twist
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