I’m far from a weapon expert, never held a real firearm in my life, but 9mm bullets are a thing, no?

I’m fairly sure there will be a lot inch-based calibers as well; but one of the most known calibers has mm literally in the name.

(and I know there are e.g. the .50 Magnum appearing in many movies/games; which is based on inch; but I would say less iconic than a 9mm; at least in the kind of movies and games I grew up with)