Why are some bullets in cal and some in mm?

The difference in measurement units for bullets—caliber (cal) and millimeters (mm)—stems from the historical development of firearms in different regions. Caliber, primarily used in the United States, typically refers to the diameter of the bullet in inches, while millimeters, commonly used in Europe and many other parts of the world, measures the diameter in metric units. This distinction reflects the cultural and manufacturing practices of each area, leading to varying standards for ammunition. Consequently, some cartridges are specified in inches (like .45 ACP) and others in millimeters (like 9mm Luger).