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Gassed! Examining Gas Operating Systems: Direct Impingement vs. Gas Piston

BY Paul Evancoe

Bohemian Karel Krnka is credited with developing the first gas-operated rifle in the late 19th century. Krnka’s early design, further improved on and perfected by Hiram Maxim and John Browning, led to today’s gas operating systems. Today, there are two gas operating system designs used in self-loading rifles and machine guns: direct impingement and gas piston. Direct impingement disciples assert that gas piston systems are heavy, less accurate and mechanically unsound. Gas piston operating system devotees claim that direct impingement systems foul easily, overheat quickly and jam often. So, exactly what is the functional difference between these two self-loading operating systems, and what are their strengths and weaknesses?

To begin, the review of a few basics might be helpful. A self-loading firearm must mechanically execute a specific set of sequential functions automatically—without user assistance—to be classified as self-loading. The sequence goes like this: The shooter pulls the trigger and the cartridge fires. The operating system needs to automatically extract that spent cartridge case from the chamber, eject it from the firearm and re-cock the hammer/striker. It must then load an unspent (loaded) cartridge from the magazine (or linked belt) into the empty firing chamber. The breech is then locked closed (the bolt is locked) and the gun is “in battery.” At this point, the weapon is ready to fire again. Both direct impingement and gas piston operating systems complete this sequence by using the high gas pressure generated by propellant (gunpowder) combustion to drive mechanical motion that, in turn, automatically cycles the action of the firearm.

What’s the difference between the two systems? For the purpose of familiarity, direct impingement will be exemplified by the AR-15, and we’ll use the AK-47 for the gas piston system.

In the 1950s, American firearms design engineer Eugene Stoner designed the AR-10 (chambered in 7.62 NATO as a competitor to the M14) and, shortly thereafter, the AR-15 (chambered in 5.56 NATO), which both employed Stoner’s unique direct impingement gas operating system. A selective fire semiautomatic/full-auto version of the AR-15, designated by the U.S. Army as the...

This article first appeared in Small Arms Review V22N8 (October 2018)
and was posted online on August 24, 2018

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