The shotshell is made up of five basic parts—one more than the rifle cartridge. Four parts of the shotshell are the same as the rifle cartridge—case, primer, powder and projectile (shot). But the shotshell has a fifth part: the wad. The job of the wad is to keep the shot charge separate from the powder.
Each gauge of shotgun requires a specific shotshell. For example, a 12-gauge shotgun requires a 12-gauge shotshell, a 20-gauge shotgun requires a 20-gauge shotshell. Make sure the gauge of your shotgun matches the gauge marked on the shotshell. The gauge is also printed on the box in which shells come from the factory.