The Winchester Model 21 is a Double-barreled shotgun

Double-barreled shotgun

A double-barreled shotgun is a shotgun or combination gun with two parallel barrels, allowing two shots to be fired in quick succession.-Construction:...

 that was produced from 1931 to 1960, and on a custom-order basis until 1993, with production continuing under license to Connecticut Shotgun Manufacturing Company.

It is not semi-automatic

Semi-automatic shotgun

A semi-automatic shotgun is a form of shotgun that is able to fire a cartridge after every trigger squeeze, without any manual chambering of another round being required...

 or pump-action

Pump-action

A pump-action rifle or shotgun is one in which the handgrip can be pumped back and forth in order to eject a spent round of ammunition and to chamber a fresh one. It is much faster than a bolt-action and somewhat faster than a lever-action, as it does not require the trigger hand to be removed from...

. It is loaded manually by lifting up the barrel and loading the shells at the breech

Breech-loading weapon

A breech-loading weapon is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel....

. This is called a break or hinge action. Like all quality doubles, production was time-consuming due to the laborious process of fitting the barrels to have an identical aim-point.