Machine Gun Recoil Systems
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Click and hold the trigger to see how a recoil-action gun fires. For simplicity's sake, this animation doesn't show the cartridge loading, extraction and ejection mechanisms.
The first automatic machine guns had recoil-based systems. When you propel a bullet down the barrel, the forward force of the bullet has an opposite force that pushes the gun backward. In a gun built like a revolver, this recoil force just pushes the gun back at the shooter. But in a recoil-based machine gun, moving mechanisms inside the gun absorb some of this recoil force.
Here's the process: To prepare this gun to fire, you pull the breech bolt (1) back, so it pushes in the rear spring (2). The trigger sear (3) catches onto the bolt and holds it in place. The feed system runs an ammunition belt through the gun, loading a cartridge into the breech (more on this later). When you pull the trigger, it releases the bolt, and the spring drives the bolt forward. The bolt pushes the cartridge from the breech into the chamber. The impact of the bolt firing pin on the cartridge ignites the primer, which explodes the propellant, which drives the bullet down the barrel.
The barrel and the bolt have a locking mechanism that fastens them together on impact. In this gun, both the bolt and the barrel can move freely in the gun housing. The force of the moving bullet applies an opposite force on the barrel, pushing it and the bolt backward. As the bolt and barrel slide backward, they move past a metal piece that unlocks them. When the pieces separate, the barrel spring (4) pushes the barrel forward, while the bolt keeps moving backward.
The bolt is connected to an extractor, which removes the spent shell from the barrel. In a typical system, the extractor has a small lip that grips onto a narrow rim at the base of the shell. As the bolt recoils, the extractor slides with it, pulling the empty shell backward.
The backward motion of the bolt also activates the ejection system. The ejector's job is to remove the spent shell from the extractor and drive it out of an ejection port.
When the spent shell is extracted, the feeding system can load a new cartridge into the breech. If you keep the trigger depressed, the rear spring will drive the bolt against the new cartridge, starting the whole cycle over again. If you release the trigger, the sear will catch hold of the bolt and keep it from swinging forward.