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Smyth Busters: The AR-15, Direct Impingement or Piston Operated?

Today, Brownells Gun Techs™ Steve Ostrem and ‪@CalebSavant‬ want to settle a debate. Or maybe just stir the pot. The debate among AR-15 fans is that the AR-15 platform actually DOES NOT have a direct impingement operating system. Steve asks the pertinent question: So where's the piston then? Caleb gets technical and starts by defining terms. A traditional GAS PISTON operating system - like on the AK series, FN FAL, HK416, or BRN-4 - sends combustion gas through a gas port in the barrel into a sleeve that applies pressure on a piston that pushes on the carrier to unlock the bolt. In a DIRECT IMPINGEMENT operating system, instead of the gas pushing on a piston, it goes down a gas tube and pushes directly on the carrier to cycle it. The AR-15 works a little differently. The gas goes THROUGH the carrier and pushes directly on the BOLT to move it. The carrier has two holes that vent the gas out of the carrier and out the ejection port. This helps minimize muzzle rise, which can be quite pronounced on a pure direct impingement system. But the big news is that the bolt acts like a piston and the carrier acts like the sleeve. So the AR-15's operating system really IS a lot like a traditional piston system! Pure direct impingement rifles like the old Swedish AG42 or French MAS-49 get a lot of carbon fouling on the carrier because the gas doesn't get vented out like it does on the AR-15. The AR-15's bolt and carrier still get dirtier than those of a traditional piston-operated rifle, which keeps all the gas (and all the fouling) up front in the gas block. So the "myth" is not busted. The AR-15 actually is not a true direct impingement rifle because the bolt itself acts as the piston and the carrier acts as the sleeve.