But did it save lives? Over the past two decades, the US National Rifle Association and its allies have carried out an ongoing campaign to discredit Australia's 1996 reforms. So it's worth reviewing what we know about the policy's impact on gun deaths.

First, there's mass shootings. Prior to the gun buyback, mass shootings were an annual occurrence. Since 1996, Australia has experienced just one event that fits the definition. In Lockhart, NSW last year, Geoff Hunt killed his wife and their three children before turning the gun on himself. The rarity of mass shootings is almost certainly a direct result of the gun buyback.

Second, there's gun homicides. In academic articles co-authored with Wilfrid Laurier University's Christine Neill, we looked at the rate at which gun homicides fell after the buyback, and at the states which saw the most significant fall. We found that the firearms homicide rate fell more sharply after the buyback, and that it fell most in states where more firearms were bought back.

Third, gun suicides. Statistically, the person most likely to kill you with a gun is yourself, so Neill and I also looked at the impact of the buyback on firearms suicide rates. Again, we found firearms suicide rates fell more rapidly after the buyback, and that there was a strong correlation between the share of weapons bought back and the drop in suicide rates. For example, the greatest reduction in weapons occurred in Tasmania, which was also the jurisdiction that saw the biggest drop in firearms suicide. Meanwhile, the smallest reduction in firearms per person was in Canberra, which also had the smallest drop in the firearms suicide rate.

Overall, Neill and I estimate that the Australian gun buyback saved at least 200 lives per year – mostly suicides. For a policy that saved this many lives, governments would typically be willing to pay at least half a billion dollars every year. Because the one-off cost of the buyback was about that much, the policy has paid for itself many times over since 1996.