Highlighted a key part for you. Australia even before the gun ban didn't have a culture of openly carrying guns in the street. We didn't have a trigger happy culture. We didn't get into an argument and shoot people. The gun buyback and ban was never intended to have anything remotely to do with the murder rate. It solved one problem and one problem only: Mass killings.
America on the other hand, ... well two of my colleagues from our Texas office got into a heated discussion on an engineering problem and ended up pulling guns on each other. The situation was de-escalated though. In the kind of culture where you reach for your piece instead of just punching a man in the face like they deserve, a gun ban may have a very different outcome.
We don't know because as you so rightly pointed out we only have data from particular cultures, very different ones to the "omg the mentally unstable need a right to bear arms too" culture.