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Click and hold the trigger to see how a gas-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.
The gas system is similar to the blowback system, but it has some additional pieces. The main addition is a narrow piston attached to the bolt, which slides back and forth in a cylinder positioned above the gun barrel. You can see how this system works in the diagram below.
This gun is basically the same as one using the blowback system, but the rear force of the explosion doesn't propel the bolt backward. Instead, the forward gas pressure pushes the bolt back. When the bolt swings forward to fire a cartridge, it locks onto the barrel. Once the bullet makes its way down the barrel, the expanding gases can bleed into the cylinder above the barrel. This gas pressure pushes the piston backward, moving it along the bottom of the bolt. The sliding piston first unlocks the bolt from the barrel, and then pushes the bolt back so a new cartridge can enter the breech.
The diagrams we've presented only depict particular examples of how these systems work. There are hundreds of machine gun models in existence, each with its own specific firing mechanism. These guns differ in a number of other ways as well. In the next two sections, we'll look at some of the key differences between various machine gun models.