Could it be that brass is dense enough compared to the medium used to measure capacity that the same volumetric change in materials produces a significantly different percentage change of weight for each of the materials?
Thought experiment: Suppose I could add brass material to the inside of a 223 case to double its weight. A change of 100% which would then cause a decrease in capacity. Does the capacity go down 100%? No, it doesn't.
But it will be proportional.
So.
Cases that are sized identically and weigh the same amount will have more consistent internal volumes than cases that aren't sorted by weight. If the outside calculated volume of the case is the same, which it will be, and case A weighs more than case B, then what contributes to the added weight?
The thickness of the brass.
So Case A will have thicker brass, but the same outside calculated volume. Which means the internal volume will be less.
Therefore sorting brass by weight is a legitimate (and time proven by thousands of targets shooters over the decades) way to sort brass for more consistent pressures.
