Ned Ludd
Silver $$ Contributor
Case weight can vary without changing the internal volume (i.e. variance in rim , extractor groove, CBTS dimension, etc.). However, when you weigh a sufficient number of cases to obtain statistically relevant values AND determine their water volume, you will generally find that case volume exhibits a linear inverse relationship to case weight, (i.e. a negative slope best straight line fit for a scatter plot of case weight versus case volume). Because of the number of "outliers" you will generally find, it's important to reiterate that drawing valid conclusions from such analyses really requires that a sufficient number of cases be examined to ensure statistical significance.
The real question as to whether sorting cases by weight is an effective method to minimize case volume variance can be answered by looking at how many outliers you have for a best line fit for a scatter plot of case weight versus case volume. In other words, how many data points are off the best fit line and how far off the line are they? A correlation coefficient of r = -1 would indicate a perfect linear relationship for case weight versus case volume. This will usually be dependent on the specific cartridge you're testing, the brand and/or Lot# of brass you're using, as well as a few other factors. The closer your individual data points lie to the best fit line, the more consistent your case volume will be for cases sorted solely by weight. The more outliers there are, or the farther the outliers are off the best fit line, the less effective sorting cases by weight will be for minimizing volume variance. However, because the outliers will be present if you do not sort cases by any method, an argument can be made that sorting cases by weight will generally improve case volume consistency as compared to not sorting them at all, even of by only a very small amount. If you really want the most consistent case volume possible, the best method is to directly measure case volume and sort them on that basis.
The real question as to whether sorting cases by weight is an effective method to minimize case volume variance can be answered by looking at how many outliers you have for a best line fit for a scatter plot of case weight versus case volume. In other words, how many data points are off the best fit line and how far off the line are they? A correlation coefficient of r = -1 would indicate a perfect linear relationship for case weight versus case volume. This will usually be dependent on the specific cartridge you're testing, the brand and/or Lot# of brass you're using, as well as a few other factors. The closer your individual data points lie to the best fit line, the more consistent your case volume will be for cases sorted solely by weight. The more outliers there are, or the farther the outliers are off the best fit line, the less effective sorting cases by weight will be for minimizing volume variance. However, because the outliers will be present if you do not sort cases by any method, an argument can be made that sorting cases by weight will generally improve case volume consistency as compared to not sorting them at all, even of by only a very small amount. If you really want the most consistent case volume possible, the best method is to directly measure case volume and sort them on that basis.
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