[spaghetti]

Circular bands probably give more power than screw-in euro bands, but in many models they require a rail to be accurate, and the rail gives a bit of drag so that power diminishes...so what?
However, open muzzles do not necessarily use circular bands: the new Omer XXV shoots with open muzzle and couple of screw-in bands, same as Seatec gabbiano open and others to mention, while the sporasub instinct has a semi-open muzzle. They're "cleaner" to aim, but power is a different issue AFAIK.
---Curiosity: here in Italy the kind of muzzle with two or more holes for circular bands is called "australian". Is it really an invention of the aussies as you guys know?