correct me if I'm wrong but your saying using a mandrel or something similar to set your neck tension to something less than what an expander ball wound can cause more runout?
It isn't about setting a particular interference, as that can just be set with a bushing. And it isn't because I like adding steps to reloading
I was THE guy who began mandrel pre-seating ~20yrs ago, and it spread onward to the crazy today.
What I was doing is fixing the problem that was pulling necks off center, contributing to RUNOUT.
I could just remove buttons, or turn them down a bit, adjust bushings, but I noticed that I could still have significant runout with that. I also noticed over time that bullet grip (neck tension) was decreasing. That if I neck sized down, with no followup expansion, necks would continue springing back outward over time. The load results would change with that. So enough expansion was needed to cause yielding(sizing), to bias spring back inward instead of outward. But that's all, just that little bit.
The runout with down sizing only of necks was due to pushing thickness variance into interference with seating bullets. I took care of this anyway with neck turning, and part of turning is mandrel expansion for fitting to a turning mandrel, and I noticed the quality of that expansion method was very good (better than button type). I could push thickness variance outward without affecting neck center.
The K&M Expandiron was fine for neck turning but too big for reloading/bullet preseating. Sinclair's mandrels were the right size, so that's what I went with. I brought this to all shooting forums of the day. Today there are all kinds of mandrel choices (that's good),, but for the wrong reason..
We don't need mandrels to set TENSION. They're not needed for that, and bushing folks know they can set tension without mandrels.
It's my contention that mandrels and expansion in general were never about that, but they're still needed.
There is an important matter and other great potentials with mandrel use, but that should be another thread.