The gas systems of these weapons is similar to that of the Swedish Ljungman M42 and the French Model 49 and 49/56 rifles, in that no piston rod is used. The gas blows directly into the bolt "bolt carrier key" (in the other rifles mentioned it blows back into the bolt carrier. This is a quote from Small Arms of the World.
Other gas operated rifles exhaust spent gas from the operating rod into the air or into the stock away from the operating mechanism. Spent gas contains carbon which is not good for a rifle. For example, The M1 exhaust gas from under the front hand guard while the M14, M1a exhaust gas into the front of the stock.
This post is simply a statement of the AR15 gas operating system.