A semi-automatic shotgun is a kind of gun. It fires one shotgun shell every time the trigger is pulled. Most shotguns, such as pump-action shotguns, lever action shotguns, and break barrel shotguns, use no energy from the last fired shell to load the next one. A person using a semi-automatic shotgun has to load the first shell themselves. The rest of the shells are loaded by the gun using part of the energy used to fire each shell.
Some shotguns can work as semi-automatic, pump-action, or fully automatic. The SPAS-12, for example, can fire in semi-automatic mode or it can use a pump action. Pump action allows less deadly projectiles such as bean bags or gas canisters to be used. Some shotguns, such as the AA-12, can be fully automatic or it can be switched to semi-automatic mode.