20gauge

20 gauge is a term used primarily to denote a shotgun bore size in the traditional gauge system. The gauge number is historically defined as the number of lead balls of the bore diameter that would weigh one pound. Therefore, a 20 gauge bore is smaller than the more common 12 gauge bores. The nominal bore diameter of a 20 gauge shotgun is about 0.615 inches (15.6 millimeters). Modern 20 gauge shells are usually 2 3/4 inches or 3 inches in length, and typical payloads range from about 3/4 to 1 ounce of shot, with common shot sizes in 7.5 to 9.

In practical use, the 20 gauge offers a balance of reduced recoil and adequate payload, making it popular for upland game hunting and clay target shooting. Its lighter weight compared to many 12 gauge firearms makes it suitable for beginners, youth shooters, and those sensitive to recoil. Many gun makers offer a wide range of 20 gauge models, from traditional sidelock and box-lock shotguns to modern autoloaders and over-unders; barrels typically range from about 26 to 28 inches.

The term can also appear in other gauge systems for different materials, such as electrical wire or sheet metal, but those gauges are unrelated to shotgun bore sizing and are measured by different standards.