 The gas is vented from the barrel and travelled via a vent through the foresight housing into the hollow front section of the piston, which causes it to move rearwards a short distance.The rear end of the piston emerges through the barrel extension to contact the forward face of the bolt carrier, causing it in turn to move rearwards.The bolt itself is of similar configuration to the AR-15 with 7 radial locking lugs engaging corresponding recesses in the barrel extension, and the extractor in place of the 8th lug.