While early machine guns relied on recoil operations, modern assault rifles and light machine guns utilize gas operation. Gas operation is a bit more complex than recoil-based systems, as it involves the firing pin striking the round, which causes the cartridge explosion, and the creation of the gas pressure. As the bullet passes the gas port, the piston starts to move back. The bullet then leaves the muzzle, while the piston starts to open the breech.

The empty case is ejected, and the run-out of gas begins. The bolt rests on the piston slide, while the gun is loaded again and the breech closed.

Blowback is another system used with self-loading firearms, and it obtains the energy from the motion of the cartridge case as it is pushed to the rear by the expanding gas created by the ignition of the propellant charge. There are actually several blowback systems with the broad principle of this operation—where each is distinguished by the methods used to control the bolt movement. It is a very simple yet effective system.

The earliest applications of blowback were the semi-automatic pistols developed in the late nineteenth century, and the cartridges of the era were well suited to such a system, as they had relatively short cases and contained low-pressure powder charges. During the First World War, the system was successfully applied to a new class of fully automatic weapons, namely the submachine gun with the introduction of the Bergmann MP18, which used a very simple cylindrical breechblock that cycled back and forth within a tubular receiver. Simple or straight blowback proved to be so reliable that was employed in such firearms as the Thompson submachine gun, MP40, Sten Gun, Uzi, and Skorpion vz. 61.

Delayed Blowback

The downside of blowback, however, was that it could not be safely used with more powerful rounds. This is where "delayed blowback" comes into play. Essentially, this system works the same way as simple blowback in that it relies on backward pressure, exerted by the powder gases onto the base of the cartridge case, pushing it backwards against the inertia of the breechblock and the force of the breechblock return spring.

The difference is that in this particular case, some mechanical or physical means are also employed to slow down—delay—the initial opening of the breech block while the bullet is still traveling through the bore and the pressure inside the bore and case is high. The concept of delayed blowback was first developed by Ferdinand Mannlicher in the 1890s, which used a "mechanical disadvantage" with low load to force ratio to slow the action's opening.

Several variants of delayed blowback have been developed including roller delayed, lever-delayed, gas-delayed, chamber-ring delayed, hesitation locked, toggle-delayed, and radial delayed. Each had particular advantages as well as disadvantages, but when it comes to blowback, simple or straight might just be the best.

Peter Suciu is a Michigan-based writer who has contributed to more than four dozen magazines, newspapers, and websites. He regularly writes about military small arms, and is the author of several books on military headgear including A Gallery of Military Headdress, which is available on Amazon.com.

Image: Reuters