Okay, so then if it's a matter of penetration a 10mm round really should do more damage than a 9mm. While the slug may not be all that much bigger, there is a good amount more powder to burn. That gives you a higher muzzle velocity and thus better penetration. It just makes no sense to me that they'd do the same damage.
I guess that's why I liked the old Underground RPG's damage. Sure it took a bit more rolling of the dice but I thought it made more sense. The wound inflicted was based on the penetration value of the round. Roll a Penetration skill check and the quality of the result gave you the type of wound. Hmmm, probably could do something similar in d20...