^ you would actually need to be concerned about case head diameter...in that expanding the bullet end of the brass far enough to fit a 9mm bullet in the 'neck' might actually give you a reverse taper...which is pretty much un-sat from a cartridge perspective.

In other words...the bullet end of the brass would be .3800 (if the brass is a similar thickness to 9mm brass).
While the case head area would only be .3759...which is smaller. Which would mean reverse taper. Now, its only 40-thousandths. which isn't THAT much.

After fire-forming, you would end up with straight wall cases. .3800 at the 'neck' and .3800 at the case head/base.