Before I discuss the Austrian buy back model, lets look at your statement "A mentally unstable person cannot go out and kill 10, 20, 30 people without the aid of semi-automatic handguns and military style assault rifles." Perhaps you forget that terrorists currently and throughout history use bombs to commit mass causality slaughter. While an assault weapon is deadly, bombs are cheaper and are capable of much more destruction and death.
The Australian buy back is another strawman argument buy the left. The Australian mandated buy back program resulted in 650,000 firearms being turned in. While the result was a reduction in gun violence, non-firearm violent crime increased; so the end result was a wash in violent crime. Australia has a population of about 20million; the US has a population of 300 million. For every 100 Australian, 20 owned guns; for every 100 Americans 80 own guns. Australia has not bill of rights or a constitutional right to own a firearm; the US has both; the right to bear arms in Australia has no where near the significance than it does in the US. There is also nothing in the Australian constitution that guarantees human rights, only five explicit individual rights in the Constitution. They are the right to vote (Section 41), protection against acquisition of property on unjust terms (Section 51 (xxxi)), the right to a trial by jury (Section 80), freedom of religion (Section 116) and prohibition of discrimination on the basis of State of residency (Section 117). Australia also has a limited separation of powers, so the government can pretty much dictate whatever it wants and the citizenry is expected to obey. The national laws of other countries are not created in a vacuum, a fact conveniently not brought up by the Left; remember how Obama praised China, "Everybody's watching what's going on in Beijing right now with the Olympics. Think about the amount of money that China has spent on infrastructure. Their ports, their train systems, their airports are all vastly the superior to us now, which means if you are a corporation deciding where to do business … you're starting to think, "Beijing looks like a pretty good option." Why aren't we doing the same thing?." If Ron Lowe thinks the US should model Australia, he best go to Australia, because it ain't happening here!