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Australia had nowhere near the saturation of gun ownership America has, and the common firearms there were simpler and cheaper. A gun buy-back would be much more expensive.

Then there's the differences in public sentiment. It's pretty obvious the American right-wing would overwhelmingly refuse to sell their guns at any price, and they represent the majority of gun owners. With current lefty distrust of police and rising extremists in the right, left wing gun owners don't seem likely to willingly disarm either. If anything I've seen an uptick in leftists arming themselves.

But even if I'm wrong, a buy-back would do nothing if we don't stop the sales of new guns first which would require an amendment to the Constitution effectively repealing one of the bill of rights. That's pretty much the highest hurdle possible in the American government.