Look, my state wants to make cold medicine prescription only. They want to do this to fight meth. Yet almost no meth today is made with cold medicine. Cold medicine is almost always bought by legal users who will never make meth. Most meth now is made by bulk chemicals brought into this country from mexico. It's easier to work with, cheaper, and less risk.
The same can be said about guns. Most users are legal and will never use their gun in a crime. Mass shoots are rare and just like drugs, if we make them illegal the users will find other means of getting their insanity in action. The only thing taking away guns will do is hurt the legal owners of guns. It will not deter crime, mass shootings, or anything else. The only way this could even start to work is if we could find a way to collect all our existing guns. In my state alone that would be impossible as there are no records of gun owners from person to person sales.
The answer here imho is to enforce existing gun laws, improve the background check process to include databases for mental illness, create a federal universal standard for CCW that is accepted in all 50 states (My state for example requires no tests, just some cash and fingerprints), and remove restrictions on conceal carry. You ever notice most mass shootings are at places and events where it is illegal to carry a gun?
Lastly, if Australian gun laws are really something the country wants to do we better start working on a constitutional amendment. Without changing the constitution any attempt at this will fail.