I would think you would know that bore is measured by a standard size gauge ball and there are also 13 and 14 sized gauge balls amoung many others. If you should look up The Worshipful Company of Gunmakers ie The London Proof House, you find in their official literature they list shotgun sizes as 12gauge 20 gauge ect.

Face it gang there is no rational for using the term bore in lieu of gauge other then you just choose to do so.