What does the term 'caliber' specifically refer to in firearms?

The term 'caliber' in firearms specifically refers to the diameter of the bore (the interior of the barrel) and the size of the cartridge that fits that bore. This measurement is crucial because it directly influences the type of ammunition a firearm can use. For example, firearms are often designated by their caliber, such as .22 or .30, indicating the bullet's diameter in inches or millimeters.

Understanding caliber is essential for safe handling and accurate shooting, as using the wrong caliber can result in improper function or dangerous conditions. In contrast, the weight of the bullet pertains to how heavy it is, which doesn't define caliber; the length of the firearm refers to the overall size, which is separate from the concept of caliber; and the type of action describes how the firearm operates, also unrelated to the direct measurement of bore diameter.