Hmmm.....I have two fluesies; a 12 and a 16. The bolt is round but the engagement surface is flat. One gun is well worn and slightly loose but I believe the culprit is the hook/pin rather than the bolt. Since the round bolt has more surface area where it interfaces with the receiver I would think that it would be more robust than a typical flat bolt. It would be interesting to know Emil Flues' thinking...