Which term describes the size of a rifle or handgun bore, as well as the size of cartridges?

Caliber refers specifically to the measurement of the diameter of the bore of a firearm, such as a rifle or handgun, as well as the size of the cartridges that are designed to be fired from it. This term is critical in firearms terminology because it helps to identify the compatibility between a firearm and its ammunition. The caliber is often expressed in either inches or millimeters and is fundamental for understanding the performance and power of different firearms.

Gauge is a term more commonly associated with shotguns, which indicates the size of the bore based on the number of lead balls of a specified diameter that can fit into the bore. Measure can refer to the act of quantifying but does not specifically pertain to firearms, while diameter is a more general term that describes the width of a circular object but does not convey the specific meaning of firearm or cartridge sizing.