The expander mandrel IS used in response to neck turning.
I use a K&M neck turning set up, of which the expander is sized about .0005" to .001" bigger then the neck turner mandrel so as to allow for brass spring back and to obtain a precise fit on the neck turning mandrel.
This expander mounts into a 7/8x14 body for use in your usaual press.
As Jerry said, after neck turning once you have selectd the appropriate bushing and run your case through the die, sizing is complete.
Do not use an expander button otherwise you will knacker up every thing you did with your neck turning and sizing.
Again just broadening on what Jerry said, on withdrawing an expander button out of a case it is possible to stretch the neck and shoulder area. I've added a couple of pics which might help [img]/ubbthreads/images/graemlins/smile.gif[/img]
Here is the expander in the foreground, with interchangeable mandrels for .224 and .300, the screws are depth adjusters.
Here is a necked turned 243.