If you're talking about Japanese police force, they usually carry the
New Mambu M60 revolver, which is based on the
Smith and Wesson Model 30. An MP9 is a sub machine gun, which are used on large scale events such as riots and even then, they'd resort to rubber pellet shotguns than sub machine guns.
If you wanna give your character some firepower I'd recommend the Glock 17 (9 parabellum rounds) or the Beretta M9 for standard usage. But if you wanna stick to the machine gun, by all means go for it, just change it the name to Sub Machine gun.
Even then I don't think police forces hand out machine guns as SOP like that, unless you isekaied from some universe where Gun Control is executed as well as the French Queen