It's beginning to sound more and more like a brass problem.
I think we have a winner!
As I mentioned, I knew I had 20 rounds of once-fired brass from Winchester factory loads. When I dug them out to throw them in the tumbler, I discovered something I'd totally forgotten -- 20 once-fired and resized Winchester cases from another box of factory loads.
These were almost certainly fired and resized before I owned a concentricity gauge, and may have been sized on my previous press, on different shellholders, or both. But I know for sure that they were sized in the same die, because it's the only 7x57 die set I've ever owned.
So, I spun them on the Case Master. Out of 20 cases, one had about .004" runout, three or four were barely nudging .003", and the rest were .002" or less (mostly much less).
Out of curiosity, I also checked neck thickness. This proved they were fired and resized before I used the Case Master, because three or four cases were uneven by more than .0015", which would have gotten them put in a cull bag if I'd been measuring. (Interestingly, none of those were in the group with higher runout, but it's not a staggeringly huge sample.)
The next step will be to resize my other 20 Winchester cases in exactly the same way I've been doing with the Remington cases that prompted this whole thread, but that's a project for a night when I've gotten over the switch to Standard Time.
