What term describes the size of a rifle or handgun bore?

The term that describes the size of a rifle or handgun bore is "caliber." This measurement indicates the internal diameter of the firearm's barrel and is crucial for understanding the type of ammunition that should be used. Caliber is typically expressed in either millimeters or inches, depending on the measurement system, and it directly influences the performance, power, and intended use of the firearm.

While "gauge" is a term used specifically for shotguns, denoting the number of lead balls of a certain diameter that would equal the weight of one pound, it does not apply to rifles or handguns. "Calibration" refers to the process of adjusting and setting up devices to ensure their accuracy and is not related to measuring barrel size. "Dimension," although it can broadly apply to any measurement of size, is too vague to specifically refer to the bore size of a firearm. Thus, "caliber" is the precise and correct term for this context.