In Fallout, ammunition modifiers are not applied properly which makes the 9mm equal to any other kind of ammunition.
In Fallout 2, some raiders in Vault 15 have H&K P90c's incorrectly loaded with 9mm ammo rather than 10mm. These weapons can be fired normally but can only be reloaded with the correct 10mm ammo. In addition, random encounter robbers carry 9mm but have the P90c loaded with 10mm. It seems that the P90 was originally going to be loaded with 9mm, but was changed shortly before release.