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Eugene Stoner's AR-10 and AR-15 use an operating mechanism that is often called "gas impingement," but which is actually a cleverly structured gas piston located within the bolt carrier. Gas is tapped form the barrel and moved all the way back to just behind the chamber, where it enters the bolt carrier. There the carrier and bolt act as the two parts of a sealed piston, and when the bolt carrier moves rearward the cam pin forces the bolt head to rotate and unlock. The expanding gas is then vented out through holes in the side of the bolt carrier. By locating the piston on the same axis as the barrel, the harmonics are improved and the overall weight of the gun can be reduced by using the gas piston elements as mass in the bolt carrier assembly. Contrary to the old adage "it shits where it eats," the operating gasses are not vented into the magazine or chamber, and the system has proven to be very reliable.

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