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Theslide on the majority offully/semi-automatic pistols is the upper part thatreciprocates ("slides") withrecoil during the gun's operating cycle. It serves as thebolt carrier group (BCG) and partly as thereceiver, and generally houses thefiring pin/striker, theextractor and frequently also thebarrel, and provides a mounting platform foriron andoptical sights.
Through the principles ofshort recoil orsimple blowback operations, the slide is moved backwards with each shot by the energy of expanding gasses caused by the combustingpropellant (often anitrocellulose-basedsmokeless powder, very rarelyblack powder). Because the slide isspring-loaded, once at the rearmost position, the spring tension will push it back towards the front. Generally, this slide movement cycle serves three functions: the extractor will empty thechamber by pulling out the spentcasing from the previous shot (which then gets removed out of the gun by theejector), the slideinertia will cock thehammer/striker to prepare for the next shot, and the bolt will push a newcartridge from themagazine into the chamber when the slide comes back forward. Thisaction cycle will be repeated for continued firing as long as the magazine contains rounds and there is nofailure to extract/eject (which causesstovepiping anddouble-feeding),failure to feed, orout-of-battery malfunction.
Fully/semi-automaticair pistols' slides operate with similar principles asfirearms, except they utilizepneumatic energy fromcompressed gas (usuallyair,CO2,propane orrefrigerants) instead of burning propellants. Because air pistols typically have much lowermuzzle energy than firearms, there is less recoil that can be used to operate the slide. However, because airgunpellets/shots have no casings that needed to be extracted (unlike firearm cartridges), a very brief slide movement is sufficient to cycle the action and load the subsequent shot, so only a small amount of energy is needed to move the slide.
On most modern designs, once themagazine is empty, aslide stop will catch and lock the slide at its rearmost position, and will only be released to move back forward after theslide release is depressed.
Having the slide automatically loading the chamber and cocking the hammer/striker with each prior shot is essential for the function ofdouble-action/single-action pistols.Hammerless andstriker-fired pistols, lacking an external hammer that can be directly manipulated, need to have the whole slide manually pulled back to cock the hammer/striker if the gun is not already cocked for firing.