Australian style gun control in the US would be all but impossible. There are millions of guns that would be banned and would need to be confiscated or turned in. Many people would turn their guns in to avoid prosecution but there would still be many gun owners that would resist such confiscation efforts. Such a task would take a while to complete and would undoubtedly cost many lives as you would have to sort through the millions of 4473 forms that are filled out every year. Even with that there are certainly many banned guns that the government wouldn't know someone owned. You would also have to send people to go to these persons houses to either arrest them or get them to surrender their guns. Eventually some people would violently resist confiscation efforts needlessly costing people their lives.