First I trim to near SAAMI minimum. A few new cases always seem to be near the minimum to start with, so this is necessary to making them uniform. I then deburr flashholes, uniform primer pocket depth, and uniform the primer pocket profile with a Wilson reamer.
Once I've got external physical consistency as close as I can, I sort the cases by weight. This is the scary part. Last time I did this with Winchester .308 cases I got weights from about 154 to 159 grains. I put the weights in Excel and made a histogram of them with 0.1 grain increments. This revealed at least four distinct peaks, from which I inferred the cases came off four sets of tooling. (I still have these sorted and will probably go back and see if head diameter or rim thickness or some other external dimension I have not altered will reveal what tooling set the individual cases came from? That would be useful to leaning which tooling overlapping cases came from?)
I next sort the cases with a NECO case gauge for wall runout back just ahead of where the pressure ring will be on firing. Neck wall runout in the .308 is virtually always half what the wall runout is back there. I usually find about 20% of Winchester cases are up the runout consistency of a Lapua or Norma case. Those get set aside for long range match loads. Last time I did this, one case showed up with 0.008" total indicated runout near the head and case wall junction. That was set aside as a curiosity. The rest (this was a batch of 500) did not exceed 0.004" TIR.
How much difference does all this make? Limiting neck runout has a measurable effect. I've had it cut group size as much as half an moa out of about 1 moa when I also used a competition seating die to keep the bullet straight. Failure to do either one negated the other, though a tool that straightens the bullet would probably substitute for using a competition seater. I can see an actual improvement in ball powder ignition consistency and group size from ball powder loads with flashhole deburring, but can measure no statistically significant difference with stick powders. I think uniforming primer pocket depth offers the real benefit of helping insure primers seat deep enough to avoid slamfires in self-loaders. So all those improvements help in some way under some circumstances.
I am not doing outside neck turning as a routine matter at this point. I may add that to my bag of tricks. One of the reasons I acquired that small Winchester batch was to have an opportunity to shoot them before and after outside neck turning to look for visible improvement. Since I am not shooting from BR chambers with narrowed necks, I am a little concerned to avoid simply making a case neck loose in a chamber.
