The New World refers to 12-gauge shotguns but they are more accurately 12-bores. The term originates from muzzle-loading days, when the diameter of a number of pure lead balls making up 1lb determined the guns diameter. So, 12 lead balls of a certain diameter would weigh a pound, giving a 12-bore or 16, 20 or 28. The .410, oddly, is measured in inches and so the term .410 gauge is correct if inelegant.
Today we think of 2-bore, 4- and 8-bores as shotguns but in the 19th century they were also used as single-ball firearms for big game. Cotton Oswell, the professional elephant hunter, used a Purdey 10-bore for his expeditions from 1844 to 1851. In 1861 it was lent to a friend for an expedition up the Nile, where it was involved in a firefight with slavers. In the mêlée, the Purdey was fired with the ramrod still in the barrel, the missile impaling the attacker.