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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Stability Formula for Aluminum Tipped Bullets (Spreadsheet linked)
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<blockquote data-quote="Michael Courtney" data-source="post: 995113" data-attributes="member: 28191"><p>You need to decide if being "more likely than not" to have an SG of at least 1.5 is good enough, or if you really need to be sure you have an SG of at least 1.5. If we were to buy a 1 in 9" Savage rifle to shoot any of the three bullets you have mentioned with a SG of 1.5 or higher, we'd have it in mind that there is a good chance we'd have to replace the factory barrel with a 1 in 8.5" or faster custom barrel to reach our goal. </p><p></p><p>When we really need to be sure, we tend to perform a "worst case" analysis rather than an analysis using the best available estimates. (In reality, there is some complex stats involved, but the approach below gives the gist in a way more readers can understand). Here is how one might run the numbers for one of our shooting ranges in South Louisiana. </p><p></p><p>Rather than use a likely atmospheric pressure, one might use a worst case pressure of 30.6 in Hg.</p><p></p><p>Temperature: 30 deg F</p><p></p><p>Bullet OAL: 1.039"</p><p></p><p>OTM depth: 0.07"</p><p></p><p>Twist rate: 1 in 9.25"</p><p></p><p>Bullet weight: 76.9 grains</p><p></p><p>Worst case SG: 1.126</p><p></p><p>Of course, once you've measured lengths and OTM depths from a couple boxes of bullets and confirmed that the barrel twist really is 1 in 9.00", one can adjust the analysis to those numbers.</p><p></p><p>OTM depths have a lot more variation than plastic tip lengths. Due to the short length and thick jacket, I think estimating the OTM depth at .177" is likely too much. The longer 75 grain Match VLD with an OAL of 1.063 might have an OTM depth close to that or longer, but I doubt that the shorter 77 grain bullet does. </p><p> </p><p>When choosing a barrel twist slower than a manufacturer's recommendation to shoot at sea level, you need to take good care in getting good inputs for the stability calculations or be prepared to buy a faster twist barrel if you get bit by Murphy.</p></blockquote><p></p>
[QUOTE="Michael Courtney, post: 995113, member: 28191"] You need to decide if being "more likely than not" to have an SG of at least 1.5 is good enough, or if you really need to be sure you have an SG of at least 1.5. If we were to buy a 1 in 9" Savage rifle to shoot any of the three bullets you have mentioned with a SG of 1.5 or higher, we'd have it in mind that there is a good chance we'd have to replace the factory barrel with a 1 in 8.5" or faster custom barrel to reach our goal. When we really need to be sure, we tend to perform a "worst case" analysis rather than an analysis using the best available estimates. (In reality, there is some complex stats involved, but the approach below gives the gist in a way more readers can understand). Here is how one might run the numbers for one of our shooting ranges in South Louisiana. Rather than use a likely atmospheric pressure, one might use a worst case pressure of 30.6 in Hg. Temperature: 30 deg F Bullet OAL: 1.039" OTM depth: 0.07" Twist rate: 1 in 9.25" Bullet weight: 76.9 grains Worst case SG: 1.126 Of course, once you've measured lengths and OTM depths from a couple boxes of bullets and confirmed that the barrel twist really is 1 in 9.00", one can adjust the analysis to those numbers. OTM depths have a lot more variation than plastic tip lengths. Due to the short length and thick jacket, I think estimating the OTM depth at .177" is likely too much. The longer 75 grain Match VLD with an OAL of 1.063 might have an OTM depth close to that or longer, but I doubt that the shorter 77 grain bullet does. When choosing a barrel twist slower than a manufacturer's recommendation to shoot at sea level, you need to take good care in getting good inputs for the stability calculations or be prepared to buy a faster twist barrel if you get bit by Murphy. [/QUOTE]
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Stability Formula for Aluminum Tipped Bullets (Spreadsheet linked)
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