As J E said, the cases must be uniform, this means that ALL prep is done prior to measuring the capacity or weight sorting. I have tried measuring capacity with once fired, and partial FL sized/trimmed, and the partial FL sized and trimmed brass gave the best results.
I use isopropal alcohol/water mixed at 50:50 to measure actual case volume.
I have moved away from weight sorting, I never found a good correlation to weight and internal capacity, some cases that were heavier actually had the same capacity as lighter cases, and the opposite was also true. I have had cases that were 5 grains outside the mean, and measured exactly the same in internal capacity. Heavier or lighter made no difference. I have seen many a case with eccentric cut extraction grooves, this alters the weight quite a bit, over 1 grain in most instances, which is outside of my mean weight of .5gr variance.
I keep my capacity to +/- 1/10 of a CC.
If you think it helps, try using a burrette, it measures in CC's and is more accurate than measuring in grains of a liquid, whether it's pure water or not. You must remember, that fired cases have residue in them, even after tumbling, this can alter the measurement, either the weight or the CC's.
It is tedious measuring, sorting and batching with a Burrette, I do it once and once only, as I know those cases shouldn't change much over their useful life.
I rarely do more than 50 cases, unless I get large swings, then a larger batch may be needed to get my comp/match brass.
Having the capacity almost the same doesn't improve accuracy that I can see on target, but, it definitely removes flyers from the group, even at 1000yrds in a highly tuned rifle.
Hope this helps.
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