Direct impingement
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A gas-operated firearm that uses direct impingement is a firearm where gas from a fired cartridge is routed directly to the bolt carrier or slide assembly to cycle the action. Unlike other gas operated firearms, direct impingement does away with a conventional gas cylinder, piston, and operating rod. The design is simpler in that high-pressure gas acts directly on the firearm's action eliminating redundant parts thereby saving weight, lowering costs, and reducing the mass of the operating parts.
The main disadvantage of direct impingement is that the breech of the firearm becomes fouled more quickly. This is caused by solids from the high-temperature gas condensing as they cool and being deposited on the bolt face and primary operating mechanism. An analogy is that steam, another hot gas, will condense quickly in air and on cooler objects. Combustion gasses contain a great deal of vaporized metals, carbon, and impurities that are in a gaseous state until they are deposited on the cooler operating parts. Thorough and frequent cleaning are required to ensure reliability.
Conversely, in a conventional gas cylinder/piston design such as the Heckler & Koch G36, most of the gas used to operate the firearm is isolated from the breech being contained within the gas cylinder and vented away from other working parts. Some alternative systems actually contain the gas completely and vent excess back into the barrel. Notable among these is the M1 Carbine and FN SCAR.
The Swedish AG-42 Ljungman was the first successful production weapon using a simplified direct-impingement operating system. Another firearm using this system is the MAS-49. The premier current example is the AR-15/M16 rifle designed by Eugene Stoner. In the Stoner system, gas is routed from a port in the barrel directly to a chamber formed in the bolt carrier. The bolt acts as the piston and is sealed with small automobile-style piston rings.
