I often wonder why shotguns are rated by "gauge" and not caliber. Here's what I found.\n\n The gauge refers indirectly to the bore diameter of a shotgun. It is not a direct measure, but is calculated as the number of lead balls of a bore diameter required to make up one pound. This is an old British measurement system. Thus, a 16-gauge shotgun has a bore diameter such that 16 lead balls of the bore diameter would weigh a pound. \n\n\nOK, who knew all this stuff?