While Google search on the terms ".410 gauge" returns nearly twice the hits as a search on ".410 bore", examination of firearms and ammunition manufacturers naming will show that they use the technically correct ".410 bore" designation. It is a bore and not a gauge because the barrel diameter is .410 inches (10.4 mm); a true gauge is a measurement of the number of lead balls of bore diameter that constitute a pound (454 gram). A true gauge measuring .410 would have a 2.25 inch (57 mm) bore; a 410 gauge would measure .225 inches (5.7 mm). However, since the larger shotgun bore sizes are measured in gauge (10, 12, 20, 16, 28), many people assume that the .410 bore should be called a "gauge" as well.