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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Twist vs Bullet Weight Question
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<blockquote data-quote="Hugnot" data-source="post: 2938418" data-attributes="member: 115658"><p>Miller thinks the weight is not enough to affect the CM, negligible mass compared to rest of bullet but length of bullet determine CP Two L values used one for CM the other for CP.</p><p></p><p>The following reference is provided.</p><p></p><p>[URL unfurl="true"]https://arxiv.org/ftp/arxiv/papers/1410/1410.5340.pdf[/URL]</p><p></p><p>"This adjustment was most reasonable,because the length in the (1 + L2) term is related tothe rotational moments of inertia. That term will bemore accurately estimated by the length of the metalpart of the bullet because the density of copper andlead are close to 10 times the density of the plastictip. Consequently, the length added by the plastic tipdoes little to change the moments of inertia. Incontrast, the other L in the formula originates in thecenter of pressure, an aerodynamic propertydetermined by the shape of the bullet without regardto material density, with a smaller effect from thecenter of gravity. Thus it makes most sense for thisto remain as the total length of the bullet."</p><p></p><p>Two bullet lengths are used one for CP, the other for CM, tip length subtracted.</p><p></p><p>Should the big pointy tip on the Barnes 120 Blackout be plastic the Miller process would subtract the tip then calculate Sg. The tip is big enough to have a big effect on CP. Got a feeling that 12 twist or slower would work fine. What are JBM 's Sg values for that bullet with a 12 & 14 twist. The huge tippy thing looks like 1/4 inch long.</p><p></p><p>Miller calculation -saw the tippy thing off, and Sg will change much, and CP will also be affected by big tip. Got to try running it.</p><p></p><p>Hornady ELDX & ELDN - no more melting metplats.</p><p></p><p></p><p></p><p>The above attached reference. Easiest to measure tip to base then subtract length of tip - commonly .12- .14 for Hornady bullets.</p><p></p><p>Also,</p><p></p><p>[URL unfurl="true"]https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi[/URL]</p><p></p><p>Compare a JBM Sg calculation using the data as shown on my Excel spread sheet. Allow some variations, like 01 or so for tip length.</p></blockquote><p></p>
[QUOTE="Hugnot, post: 2938418, member: 115658"] Miller thinks the weight is not enough to affect the CM, negligible mass compared to rest of bullet but length of bullet determine CP Two L values used one for CM the other for CP. The following reference is provided. [URL unfurl="true"]https://arxiv.org/ftp/arxiv/papers/1410/1410.5340.pdf[/URL] "This adjustment was most reasonable,because the length in the (1 + L2) term is related tothe rotational moments of inertia. That term will bemore accurately estimated by the length of the metalpart of the bullet because the density of copper andlead are close to 10 times the density of the plastictip. Consequently, the length added by the plastic tipdoes little to change the moments of inertia. Incontrast, the other L in the formula originates in thecenter of pressure, an aerodynamic propertydetermined by the shape of the bullet without regardto material density, with a smaller effect from thecenter of gravity. Thus it makes most sense for thisto remain as the total length of the bullet." Two bullet lengths are used one for CP, the other for CM, tip length subtracted. Should the big pointy tip on the Barnes 120 Blackout be plastic the Miller process would subtract the tip then calculate Sg. The tip is big enough to have a big effect on CP. Got a feeling that 12 twist or slower would work fine. What are JBM 's Sg values for that bullet with a 12 & 14 twist. The huge tippy thing looks like 1/4 inch long. Miller calculation -saw the tippy thing off, and Sg will change much, and CP will also be affected by big tip. Got to try running it. Hornady ELDX & ELDN - no more melting metplats. The above attached reference. Easiest to measure tip to base then subtract length of tip - commonly .12- .14 for Hornady bullets. Also, [URL unfurl="true"]https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi[/URL] Compare a JBM Sg calculation using the data as shown on my Excel spread sheet. Allow some variations, like 01 or so for tip length. [/QUOTE]
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Twist vs Bullet Weight Question
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