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Rifles, Reloading, Optics, Equipment
Reloading
SD's and ES's??????? Help!!
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<blockquote data-quote="Pdvdh" data-source="post: 428767" data-attributes="member: 4191"><p>Most all can readily understand average MV and Extreme Spreads in measured muzzle velocity. But how does one explain the uniformity or the dispersion of MV? If the ES is 50fps, how many bullets out of 100 fired leave the muzzle with velocity that varies by 10 fps, 20 fps, 30 fps, or 40 fps? </p><p></p><p>That's what Standard Deviation (SD) provides. It's a measure of the uniformity of your MVs. Most chronographs will calculate SD these days. If you have an ES of 50, you don't know how many bullets out of 100 fired will leave the muzzle with that difference of 50 fps. If you shoot over your chronograph during load development, by the time you're done, you'll probably have enough rounds fired to determine a reasonably correct SD. An SD of 15 means that 2/3's of your bullets will leave the muzzle within 15 fps of your average (mean) MV. </p><p></p><p>Example using this data set:</p><p><u>Average MV = 3000</u> fps. <u>ES = 30</u> fps. <u>SD = 10</u> fps.</p><p>The SD of 10 means that 2/3 of your shots should be expected to have a MV between 2990 and 3010 fps. </p><p>Which also means that 1/6th of your shots should be expected to have a muzzle velocity between 2895 and 2990 fps.</p><p>And the remaining 1/6 of your shots should be expected to have a muzzle velocity between 3010 and 3015 fps.</p><p></p><p>I can usually get SDs down to 15 fps or less with some load development time and my goal is an SD of 10 or less. Any SD of less than 10 fps is pretty darn good for a large capacity rifle cartridge like the 7mm Rem Mag or 300 Win Mag. The smaller the better - same as with group measurements. However many consider any single digit SD to indicate a very acceptably uniform load with 7mm Rem Mag and 300 Win Mag class cartridges for purposes of shooting long range (~1000 yds).</p></blockquote><p></p>
[QUOTE="Pdvdh, post: 428767, member: 4191"] Most all can readily understand average MV and Extreme Spreads in measured muzzle velocity. But how does one explain the uniformity or the dispersion of MV? If the ES is 50fps, how many bullets out of 100 fired leave the muzzle with velocity that varies by 10 fps, 20 fps, 30 fps, or 40 fps? That's what Standard Deviation (SD) provides. It's a measure of the uniformity of your MVs. Most chronographs will calculate SD these days. If you have an ES of 50, you don't know how many bullets out of 100 fired will leave the muzzle with that difference of 50 fps. If you shoot over your chronograph during load development, by the time you're done, you'll probably have enough rounds fired to determine a reasonably correct SD. An SD of 15 means that 2/3's of your bullets will leave the muzzle within 15 fps of your average (mean) MV. Example using this data set: [U]Average MV = 3000[/U] fps. [U]ES = 30[/U] fps. [U]SD = 10[/U] fps. The SD of 10 means that 2/3 of your shots should be expected to have a MV between 2990 and 3010 fps. Which also means that 1/6th of your shots should be expected to have a muzzle velocity between 2895 and 2990 fps. And the remaining 1/6 of your shots should be expected to have a muzzle velocity between 3010 and 3015 fps. I can usually get SDs down to 15 fps or less with some load development time and my goal is an SD of 10 or less. Any SD of less than 10 fps is pretty darn good for a large capacity rifle cartridge like the 7mm Rem Mag or 300 Win Mag. The smaller the better - same as with group measurements. However many consider any single digit SD to indicate a very acceptably uniform load with 7mm Rem Mag and 300 Win Mag class cartridges for purposes of shooting long range (~1000 yds). [/QUOTE]
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SD's and ES's??????? Help!!
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