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Click and hold the trigger to see how a blowback-action gun fires. For simplicity's sake, this animation doesn't show the cartridge loading, extraction and ejection mechanisms. See the "Machine Gun Feeding: Belt System" section to find out how these components work.
A blowback system is something like a recoil system, except that the barrel is fixed in the gun housing, and the barrel and bolt don't lock together. You can see how this mechanism works in the diagram below.
This gun has a sliding bolt (3) held in place by a spring-driven cartridge magazine (5), and a trigger mechanism (1). When you slide the bolt back, the trigger sear (2) holds it in place. When you pull the trigger, the sear releases the bolt, and the spring drives it forward. After the bolt chambers the cartridge, the firing pin sets off the primer, which ignites the propellant.
The explosive gas from the cartridge drives the bullet down the barrel. At the same time, the gas pressure pushes in the opposite direction, forcing the bolt backward. As in the recoil system, an extractor pulls the shell out of the barrel, and the ejector forces it out of the gun. A new cartridge lines up in front of the bolt just before the spring pushes the bolt forward, starting the process all over again. This continues as long as you hold the trigger down and there's ammunition feeding into the system.