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The slide is the part on a majority of semi-automatic pistols that moves during the operating cycle and generally houses the firing pin/striker and the extractor, and serves as the bolt carrier group (sometimes referred to as a bcg). It is spring-actuated and once it has moved to its rearmost position during the firing cycle, spring tension forces it back to the starting position. While the slide is propelled forward from the force of the recoil spring, it chambers a fresh cartridge—unless the firearm has a slidestop designed to capture the slide in its most rearward position, while the magazine is not empty (thus allowing a quicker reload without having to manually pull the slide back).
Through the principles of recoil or blowback operation, the slide is forced back from the energy of expanding gasses caused by the propellant (often a cellulose compound, or black powder) combusting with each shot. Generally, this action serves three purposes: ejecting the spent casing, cocking the hammer or striker for the next shot, and loading another cartridge into the chamber when the slide comes forward.
On most designs, once the magazine and chamber both are empty, the slide will lock back, released only when the slide stop is depressed; if a new magazine is inserted before the slide stop is depressed then a new cartridge will be chambered.
Automatically cocking the hammer or striker is an important function of double-action / single-action pistols. However, some semi-auto pistols are double-action only, and have no sear notch for single-action operation.