.177 calibre
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.177 calibre (4.5mm) is the smallest size of pellets widely used in air guns, and is the only caliber generally accepted for formal target competition. It is also sometimes used for hunting small game, and in field target competition, where it competes with .20 calibre (5mm) and .22 caliber (5.5mm) rifles.
Steel BB shot is .175 inch diameter. Some air guns are designed to accept .177 pellets, or .177 lead shot, or .175 steel BBs interchangeably.
.17 calibre (actually .172 inch diameter) is the smallest size bullet that is widely available for use in firearms, both in rimfire (17 HMR) and centerfire (17 Remington) ammunition. Production of .14 calibre barrels, rifles and bullets is a cottage industry in the USA, while .12 and .10 calibre rifles have been made on an experimental basis.
See cartridge for a discussion of the recommended uses of most sizes of ammunition.