What does “caliber” indicate concerning firearms?

Caliber refers to the diameter of the firearm's barrel, which directly corresponds to the size of the ammunition used in that firearm. When discussing firearms, caliber is expressed in inches or millimeters, and it is an important specification that indicates the appropriate ammunition type that can be used safely and effectively in a specific firearm. For example, a firearm designated as .45 caliber has a barrel diameter of .45 inches and is designed to use bullets that are compatible with that size. Understanding caliber is crucial for selecting the correct ammunition to ensure proper firearm function and safety.