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One mathematically unsophisticated writer decreed that Standard Deviation WAS 1/3 the Extreme Spread.
A mathematically sophisticated scientist who got into shooting suggested that Coefficient of Variation, the SD as a percentage of the Average, was a better measure of consistency. He found that 1% pistol ammunition was about what could be done with careful use of normal reloading gear; CoV 1% = 8 fps SD on an 800 fps load.
But your highest velocity shot and your lowest velocity shot will normally shoot the fartherest apart in elevation at long range, so those are what you need to know. (Unless you shoot the fabled match conditioned SMLE which "compensates" at 600 yards.)
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