What is the bore diameter of a shotgun referred to as?

The bore diameter of a shotgun is referred to as "gauge." This term specifically denotes the size of the shotgun's bore in relation to the number of lead balls of a specific diameter that equal one pound. For example, a 12-gauge shotgun has a bore diameter such that 12 lead balls of that diameter weigh one pound. This measurement system is unique to shotguns, whereas caliber is used in rifles and handguns, indicating the diameter of the bore in inches or millimeters.

Other options, like bore size and choke, do not accurately represent the measurement terminology for shotgun bores. Bore size could refer to specific diameters but is not the standardized term used. Choke refers to the constriction at the end of the shotgun barrel that affects shot spread, rather than the actual bore diameter itself. Thus, gauge is the correct term since it directly specifies the measurement system related to shotgun bores.