I'd guess that cylinder could be used to control the cycle rate on the automatic version. Not sure what purpose it serves on the semi-auto. Maybe an expert will chime in here?septo said:I think it does that to slow the extraction of the round. Moves I mean. I don't get the point of the metal cylinder that slides and is attached to the bolt. It seems it would accelerate the foces on the bolt as the cylinder slams back and forth. Am I right?