What term describes the diameter dimension of a bullet or the barrel of a firearm?

The term that describes the diameter dimension of a bullet or the barrel of a firearm is "caliber." Caliber refers specifically to the measure of the internal diameter of the firearm's barrel and, correspondingly, the diameter of the ammunition it fires. This measurement is crucial for identifying the type of ammunition that can be safely used in a particular firearm, as well as for assessing the firearm's performance characteristics.

In contrast, gauge is primarily used in reference to shotguns and describes the number of lead balls of a specific diameter that can fit into the barrel. Size and width are more generic terms and do not specifically relate to firearms terminology. Thus, caliber is the most accurate and specific term for describing the diameter dimension in this context.