The 20 gauge shotgun, also known as 20 bore, is a type of smoothbore shotgun. 20 gauge shotguns have a bore diameter of .615 in (15.6 mm), while the 12 gauge has a bore diameter of .729 in (18.5 mm). 12 gauge and 20 gauge shotguns are the most popular gauges in the United States. The 20 gauge is popular among upland game hunters, target shooters, and skeet shooters.
3 23⁄4" 20 gauge shells (right) loaded with #71⁄2 birdshot.
The gauge of a firearm is a unit of measurement used to express the inner diameter of the barrel.
From left to right; a .45 ACP cartridge, a .410 bore shotshell, a 20-gauge shotshell, and a 12-gauge shotshell
left-to-right: .410 bore, 28 gauge, 20 gauge, and 12 gauge shotgun shells
Garden gun calibers: 9mm Flobert shot, 9mm Flobert shot, .22 Long Rifle shot, .22 Long Rifle, .22 Long Rifle shot, .22 CB Short, and 9mm Flobert BB cap
A 10-gauge (3+1⁄2-inch (89 mm)) shotgun shell shown next to a United States quarter