I have assembled it and it seams to work fine.I was sitting on my bench chair facing away from it when i pulled the barrel i heard something hit the floor about 2 ft in front of me when i looked that ring was on the floor i think it might have come out of where the piston engages the gas exit port on the barrel it seems to fit in that tube here are some pics[ATTACH=CONFIG]980[/ATTACH][ATTACH=CONFIG]981[/ATTACH]