I use the Expandiron prior to turning the necks for my 6MM BR & 223 brass. I can't say that it has an effect on runout one way or the other.

Rick

It will increase runout if you don't use the threaded shank that comes with it,..to set the web to stop the press ram when bottoming out before the case mouth hits the body that holds the expander. Other than that,..I see no real difference.

Do you like the expandiron better than the expander button that is in the FL sizer dies (RCBS)?

I use a similar expander mandrel setup from Sinclair and mainly use it to make sure the necks fit the neck turning mandrel properly.....as for the in die expanders....I use a polish on the expander ball and use "O" rings under the threaded collar that holds the expander/decapp assy and this makes the brass slide over the expander and lets the expander ball center itself in the case neck....with good straight cases with good neck uniformity and good interior neck wall lube and smoothness , I get good results with the factory die expander in the die..after the polish and floating.....with unequal wall thickness in the necks and rough interior neck wall surface the runout is much worse.......hth..good luck and good shooting-loading!!

The expandiron is matched to their mandrels for neck turning. The diameter of the expandiron is such that when it is used to open the case neck, and the spring-back of the brass, it will fit the mandrel very well. Some expander buttons may work too, it all depends on the diameters of the expander and the mandrel. I would look elsewhere to find a runout problem, not in the expanderiron.