Direct impingement
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A gas-operated firearm that uses direct impingement is a firearm where gas from a fired catridge is routed directly to bolt carrier or slide assembly to cycle the action in the firearm. Unlike other gas operated firearms, the direct impingement system does away with a gas cylinder, piston, and operating rod. The design is simpler in that the gas acts directly on the firearm's action.
The first instance of direct impingement gas operated firearm was the is the Swedish AG-43 Ljungman. The main current example is the AR-15/M-16 rifle designed by Eugene Stoner. Gas is routed from a port in the barrel via a narrow tube directly to the bolt carrier. In a tradional gas operated design, such as the AK-47, FAL, and M1 Garand, propellant gas is trapped in a cylinder where it forces a piston to operate the action.
A disadvantage of the direct impingement system is that the breech of the firearm, where most of the critical moving parts reside, can become more susceptible to powder fouling. Reliability can be compromised if the firearm is not cleaned often. In the gas cylinder/piston design, most of the hot gases and potential for fouling are isolated from the breech and contained within the gas cylinder.