Let me run this by you guys. I have been doing it for years, but seldom see referance to it, anywhere on the 'net.
First, you need a piece of drill rod just a little longer than the overall length of the case in question. One end flat and the other turned to a point on a lathe. Next, you deburr the flash hole.
It doesn't matter which case you select as your baseline. Just measure it and re-zero your dial caliper. This case will be zero. It might, in fact be 2.355" but it doesn't matter, I just change the dial to indicate zero. I made up a board with divisions the whole length, in thousanths, plus and minus, from the center. This board is about five feet long. Now, I measure every case. if it is .005" under, it goes in that (-.005) column. A case that might be .003" over would go to the (+) other side of zero. What I'm measuring is the web thickness, and it gives an indication of internal volume, since this portion of the case shows substantial variation, much more than a thou difference in neck thickness.
Anyway, when I'm finished, and done with all the rest of the culling and separating, I divide my cases thus:
I save the ten thickest and ten thinnest for load development. Then I scratch a mark on the base, to the right or left or exactly in the middle of the headstamp. Generally, for instance, I wind up with 40 low and 40 high, and 100 right in the middle. Equal thirds would probably be about the same? Anyway, it seems that I get more consistant velocities, all things being equal. I dump them all into the tumbler, and they all get the same load, but I put them in separate boxes. Accuracy is a tossup, but the velocity does show a slight difference, not sure if it would be significant for everybody.
Jack and Dave, both of you guys have been around. Have you tried this, have you heard of it, and what might be your unvarnished opinion? You know, as in gambling; everybody has a system that doesn't work! I can take it. lol