I tried a RCBS 38 Cowboy expander plug in my 9mm die. Besides being too long to flare the mouth, it was only .3564 in diameter. .358 bullets swaged to .356 in the case.
Heavy leading resulted in my .357 bore. (Bullets were unsized 3 week old air cooled COWW equiv., lubbed with WLL)
Should I try a plug from a .38 S&W? It's supposed to be .361. I suppose I could hone it to .358, if need be...
I left the cases uncrimped. Cartridges easily dropped into the chamber.