Final answer:
The calibers of the bullets .40 caliber, .45 caliber, and 9 mm refer to their respective bullet diameters.
Step-by-step explanation:
The calibers of the bullets .40 caliber, .45 caliber, and 9 mm refer to the diameter of the bullets. Caliber is typically measured in inches or millimeters. .40 caliber refers to a bullet with a diameter of 0.40 inches, .45 caliber refers to a bullet with a diameter of 0.45 inches, and 9 mm refers to a bullet with a diameter of 9 millimeters.