A 12-gauge shotgun is one whose barrel is the diameter of a ball of lead weighing one-twelfth of a pound. The diameter of a 12-gauge barrel is approximately .729 inches.Know More
Gauge as a measurement of barrel width is an artifact of the age of the cannon, when guns were measured by the weight of the shot they fired. Over time, larger gauges have fallen out of common use, leaving 12-gauge and smaller shotguns the most common shotguns available. As of 2014, the 10-gauge is the only larger shotgun in common use, with 20-gauge and 28-gauge shotguns offering a smaller barrel width and less recoil than 12-gauge models.