Semi-automatic firearms use many different mechanisms for cycling the action (the mechanism that moves ammunition through a firearm). Blowback-operated firearms are among the least complex. Other operating principles for self-loading firearms include blow-forward, gas-operation and recoil-operation.
Blowback is a system of operation for a self-loading firearm that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gas created by the ignition of the propellant charge. A blowback firearm features an unlocked breech. The breech is the rear of the barrel that opens into the chamber. When the slide (the part that reciprocates to feed ammunition through a firearm) moves into its most forward position, it’s referred to as being “in battery.” An unlocked breech slide does not lock to the barrel when fired.
A blowback firearm uses force created by burning gunpowder to cycle the action. The gas from a blowback-operated pistol pushes the bullet down the barrel when fired. As the bullet moves forward, the slide blows back under the pressure of the fired round. After the pressure drops to a safe level, the breech moves backward, extracting and ejecting the case. Once the case ejects, the slide moves forward, stripping another round off the top of the magazine, back into battery. The cycle is then ready to start again.
A blowback-operated firearm typically lacks a mechanical lock between the breech and the barrel. This makes it simpler to design and cheaper to build. A gas- or recoil-operated firearm requires a more complex system to lock and unlock the bolt. A blowback-operated firearm is quite reliable. This is because of the forcefulness of its ammunition being fed (extraction keeps it working consistently).
Several blowback systems exist within this broad principle of operation. Each is distinguished by the methods used to control breech movement. A few locked-breech designs use a form of blowback — such as the primer actuation — to perform the unlocking function. Delayed blowback or retarded blowback is a subclass of blowback operation.
A recoil-operated firearm uses the energy of recoil to cycle the action. It is one of the earliest and most successful methods of operation for an automatic firearm. It works for a broad spectrum of weapons — from pocket-sized pistols up to heavy machine guns and automatic cannons.
Hiram Maxim, an American inventor living in Europe, was probably the first person to utilize the energy of recoil to automatically cycle the action of a gun, receiving a patent in 1884. The successful design quickly gained acceptance with firearm designers in many countries, including Hugo Borchardt (1893), the Federle brothers (1894) and John Browning (1896).
Both direct impingement and gas piston are systems used to fire a round, eject the spent cartridge and cycle through to chamber a new round. These systems use the force of the gas created from the burning propellant to force the bolt carrier group back through the ejection and reloading action.
A gas-operated firearm is one that uses the energy from a portion of high-pressure gas from the cartridge being fired to work the action in semi-automatic and fully automatic guns. One of the earliest gas-operated designs belongs to the French Clair brothers (Jean Baptiste, Benoit and Victor), who patented a gas-operated rifle in the 1890s.
The major difference between a direct-impingement firearm and a gas-piston-operated firearm is where the gas goes after helping to expel from the barrel. In a direct-impingement gas system, the gas created from firing gets pushed along the barrel with the bullet until it hits the gas block. The bullet will then leave the barrel, but the gas is forced up into the gas tube and directed back along the length of the barrel and into the gas key. At this point, the gas forces the bolt carrier group back to eject the spent case and load the next round.
A gas piston cycles the action in the same manner. But the notable difference is that the gas never cycles back to the carrier group. Instead, when a shot is fired, the gas forces the bullet down the barrel and reaches the gas block. Rather than traveling along the gas tube and back into the chamber, the gas stops just above the gas block, hits a solid piston and expels out at that point. The gas still goes up and out of the barrel with the same amount of force. Therefore, it still has the power to cycle the bolt carrier group just as effectively.
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