That was simply a matter of Political will and a compliant population. The US doesn't have that option. 

Once the legislation was passed in Australia banning semi automatic rifles, the ones already on the street were bought back (vs confiscated) and people willingly complied. I believe somewhere around 650,000 were turned in.

Now, lets look at the US problem.
Americans deeply believe that their right to bear arms, as protected in the Constitution and upheld numerous times in the Supreme Court, is their rightful defense against the State or Government.
As such, Americans have now amassed an arsenal of over 300,000,000 guns w/ over 1 trillion rounds of ammo. Do you think any Government is going to take it away?