The AR Is Not Direct Impingement

Direct impingement is where the gas bled from the barrel is fed through a tube to strike the face of the bolt carrier, causing it to move to the rear and open the bolt. As far as I am aware, there have only been two production designs that used direct impingement: The French MAS 49 and Swedish Ljungmans AG m/42 (which spawned the Egyptian Hakim and Rashid).

You can see in the Ljungmans patent drawing that gas will travel down the tube (10) and strike the bolt carrier at (12).

In the AR system the gas is fed inside the bolt carrier through the gas key. There is an expansion chamber inside the carrier. With the bolt closed, the rear of the bolt is inside this chamber. When gas enters the chamber it forces the bolt out. Since the bolt cannot go forward because it is held by the rear of the barrel, the bolt carrier goes to the rear.

The rear of an AR bolt is really a piston!

The confusion between these systems stems from the gas tube being placed in-line with the bolt carrier. Gene Stoner's original prototypes had the gas tube running along the left side of the rifle and entering the expansion chamber through a hole in the side of the bolt carrier.