What Is The 'Bore' Of A Rifle, Like 12 Bore?

In the case of smoothbore guns such as muskets and shotguns the calibre of a gun was referred to as its bore. This term bore was used instead of the term gauge and referred to the number of balls present in each pound of lead. It was only later that this started being referred to in terms of millimetres or inches. The term is used in the case of mostly shotguns as in a 12 gauge or a 20 gauge etc. Smoothbore refers to guns that have smooth barrels; without any riffled groves. The rifling in barrels was provided to stabilize the flight of the projectile. The rounds that were fired from smooth bore were usually provided with some sort of fin stabilization to prevent them from tumbling during flight.