Most American ammunition is still non-metric. The NATO versions of military ammo like 5.56mm and 7.62mm are known in America as .223 Remington and .308 Winchester respectively.
9mm was not an American round. 9mm Parabellum was originally designed by Austrian firearm designer Georg Luger in 1901.
What American truly did when it came to converting to metric was to start using it in automotive and other machinery for bolt/nut sizes. I've hated it ever since about 1976 or so.
And the only reason the auto manufacturers did this was in the interest of their longtime goal to make repairing your own auto impossible due to proprietary parts, manuals, and required "specialty" tools; thus forcing the auto owner to take their vehicle to the "authorized" dealer for repairs.
9mm was not an American round. 9mm Parabellum was originally designed by Austrian firearm designer Georg Luger in 1901.