Actually, if it's an 1880 gun, the "fractional" bore sizes--which give you a much better idea of the gun's early bore diameter (12/1, 13/1 etc) than does just "12 gauge"--were not yet in use. Although the standard bore diameter for a 12 gauge in 1880 was .729, just like it is now, the next larger bore (an 11) had a standard measurement of .751. Therefore, a 12ga gun made in 1880 and not reproofed since then could have had a bore diameter a good bit larger than .729 when it came from the maker and passed proof.