Lyman M die 223 rem

As someone that has a Lyman m-die in 223, I'm not really getting what you're saying, here. The m-die doesn't do the full expansion(technically, neither does the expander ball, but let's ignore that for now). The idea of the m-die is basically just to flare the neck to make bullet seating easier, primarily with bullet feeders(at least that is how mine works, and I doubt they redesigned it in the last couple years). Realistically, I don't see any reason you couldn't just swap out an expander ball with a universal decapping pin(Lee dies would make this very easy), then use the m-die to flare the neck and let the bullet do the rest of the expanding. I wouldn't imagine it would be enough neck tension to crush shoulders or damage the bullet, but I can't say for certain(and don't feel like adjusting all the dies in my press to find out).

Personally, I would just use a normal resizing die like normal, then the m-die. That is how I have been using mine, and haven't had significant enough issues with concentricity to cause any noticeable effect on performance.

Yes when using an m die expanding happens in a second step after sizing, with expander ball removed. This is the same as any expanding with a mandrel or a two step like the M or the NOE expanders.

The Redding S die can be used with & without the expander, & still decap.

The M die is for cast bullet seating.

For jacketed bullets, many now use this

Who’s die do you have?

If you also have a pistol die set from the same manufacturer, you might check and see if you can swap out the expanding decap pin for one that doesn’t expand at all (straight walled decap pins don’t expand or they would get locked into the die).

That said, that won’t ensure to give you 0.000” run out.

I think the best use of a run out fixture is to allow you to test a fired case, each step along the way, and see if and where error is introduced.

IF it’s OK after firing but right out of the size die, has run out. The first thing I would do is to remove the decap/expander completely. IF it doesn’t induce run out at that point, you a have found your problem.

Now, I would explore case lubes and application of them in the neck of the case and repeat until sized cases are not being yanked out of round.