Direct impingement

Direct impingement is a type of gas operation for a firearm that directs gas from a fired cartridge directly into the bolt carrier or slide assembly to cycle the action.

Direct impingement tends to be lighter and shorter due to its method of operation, but exchanges this for being dirtier due to exposing more moving parts to fouling from propellant gases and potentially less reliable as a result.

Unlike conventional gas-operated firearms, direct impingement does away with a separate gas cylinder, piston, and operating rod assembly. High-pressure gas acts directly upon the bolt and carrier thereby saving weight, lowering costs, and reducing the mass of the operating parts, and thereby the wear on mechanical parts due to movement. By removing the gas piston, the potential amount of moving mass is lowered, thus decreasing the potential for a disrupted sight picture, as caused by the mass of the piston adding to the disruptive mass.

The main disadvantage of direct impingement is that the breech of the firearm's firing mechanism becomes fouled more quickly due to being exposed to the propellants of the cartridge. This is caused by solids from the high-temperature gas condensing on the bolt face and primary operating mechanism. The combustion gases contain vaporized metals, carbon, and impurities in a gaseous state until they contact cooler operating parts. The deposits increase friction on the bolt's camming system leading to malfunctions, so that thorough and frequent cleaning is required to ensure reliability. The amount of fouling depends upon the rifle's design as well as the type of propellant powder used. For example, the French MAS 44 and MAS 49 series of rifles was known to have been successfully operated for years with corrosive-primed ammunition using ordinary field cleaning expedients such as gasoline (as solvent) and straight-grade motor oil (as lubricant)..

A further disadvantage of direct impingement is that combustion gases heat the bolt and bolt carrier as the firearm operates. This heating causes essential lubricant to be "burned off". Lack of proper lubrication is the most common source of weapon malfunctions. These combined factors reduce service life of these parts, reliability, and mean time between failures.

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