Relevant to the Australian gun debate is a study conducted by the Pittsburgh Graduate School of Public Health, which has that four out of five US criminals obtain their guns illegally:
http://www.breitbart.com/big-government/2016/07/29/study-four-of-five-criminals-acquire-guns-illegally/
The study examined guns recovered at Pittsburgh crime scenes in 2008. One in five guns were used by people who had no prior criminal record and thus were able to legally acquire a gun.
An article “Young, Dumb, Armed” at http://www.theage.com.au/interactive/2016/gun-city/day1.html, tells us that although Australians have been disarmed, Melbourne (and probably Sydney) is a gun city, awash in illegal guns.