The gauge of a firearm is a unit of measurement used to express the diameter of the barrel. Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm, and is expressed as the fraction of a pound (e.g. a 1/12th pound ball fits a 12-gauge bore). The term is related to the measurement of cannon, which were also measured by the weight of their iron round shot; an 8 pounder would fire an 8 pound (3.6 kg) spherical cast iron ball and had a bore diameter of about 91 mm (3.6 inches).
Commonly used today in reference to shotguns, though historically it was also used in large double rifles, which were made in sizes up to 4 bore during their heyday in the 1880s, being originally loaded with black powder cartridges. These very large rifles, sometimes called elephant guns, were intended for use in India and Africa for hunting dangerous game.
gauge and bore are interchangeable terms. what i am asking is what year was the 3 inch magnum chamber introduced. I know that the standard was always 2 3/4"
Not sure of the exact year, but my best guess would be around 1917-1920. Fox, LC Smith and Parker guns could be had with 3" chambers as early as 1921/1922.