What caliber of ammunition does the M9 use?

The M9, which is a semi-automatic pistol widely used by the U.S. military, is specifically designed to fire 9 mm ammunition. The designation ".45 ACP" corresponds to a different caliber typically associated with larger handguns, and the 7.62 mm and 5.56 mm are rifle calibers used in different contexts entirely, often for military rifles and carbines.

The use of 9 mm ball ammo allows for a combination of manageable recoil and effective ballistics, making the M9 suitable for various tactical situations. This characteristic makes 9 mm a popular choice for many law enforcement and military applications, reinforcing the reasoning behind the M9's standardization around this particular caliber.