Australia has a murder rate of 1/100,000. According to your source, the homicide rate of the US is 5.5 according to your source, and the non-firearm homicide rate in the US is 1.8 [2]. By contrast, the total homicide rate in Australia is only 1.3.
I.e., with or without guns, the US is a more violent place than Australia. Drawing conclusions about gun control without correcting for this fact is a fallacy.
Incidentally, how do you define a weapon that is "designed purely for mass killing"?