.177 caliber

.177 caliber (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 caliber (5 mm) and .22 caliber (5.5 mm) rifles. Compared with a .22 pellet, the .177 travels faster and on a flatter trajectory. This is the reason for it being used in target competitions as the competitor does not have to adjust for drop very much. In hunting, the .177 is in general inferior to .22 caliber pellets as it is smaller thus causing less impact damage. Heavier pellets can however increase its effectiveness.

Steel BB shot is 0.175-inch (4.4 mm) diameter. Some air guns are designed to accept .177 pellets, or .177 lead shot, or .175 steel BBs interchangeably.

The .17 caliber (actually 0.172-inch (4.4 mm) or 4.37 mm 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 caliber barrels, rifles and bullets is a cottage industry in the USA, while .12 and .10 caliber rifles have been made on an experimental basis.