Slide action

A pump-action or slide-action firearm is one in which a forend can be moved forward and backward 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 the trigger while reloading. Once fired, the forend is slid rearward by hand and the expended cartridge ejected. It is then reloaded by manually moving the forend to the front.

The first slide action patent was issued to Alexander Bain of Britain in 1854.

Older pump-action shotguns are often faster than modern semi-automatic shotguns, as they often did not have a trigger disconnector, and were capable of firing a new round as fast as the pump action was cycled, with the trigger held down continuously. This technique is called a slamfire, and was often used in conjunction with the M1897 in the First World War's trench warfare.

Modern pump-action designs are a little slower than a semi-automatic shotgun, but the pump-action offers greater flexibility in selection of shotshells, allowing the shooter to mix different types of loads and for using low-power or specialty loads. Semi-automatic shotguns must use some of the energy of each round fired to cycle their actions, meaning that they must be loaded with shells powerful enough to reliably cycle. The pump-action avoids this limitation. In addition, like all manual action guns, pump-action guns are inherently more reliable than semi-automatic guns under adverse conditions, such as exposure to dirt, sand, or climatic extremes. Thus, until recently, military combat shotguns were almost exclusively pump-action designs.

The cycling time of a pump-action is quite short. The manual operation gives a pump-action the ability to cycle rounds of widely varying power that a gas or recoil operated firearm would fail to cycle, such as most less-than-lethal rounds. The simplicity of the pump-action relative to a semi-automatic design also leads to improved durability and lower cost. It has also been noticed that the time taken to work the action allows the operator to identify and aim on a new target, avoiding a "spray and pray" usage.

...
Wikipedia