Australia.
"... the Australian government banned automatic and semiautomatic firearms, adopted new licensing requirements, established a national firearms registry, and instituted a 28-day waiting period for gun purchases. It also bought and destroyed more than 600,000 civilian-owned firearms, in a scheme that cost half a billion dollars and was funded by raising taxes. The entire overhaul, Friedman pointed out, took just months to implement."
"...The number of mass shootings in Australiadefined as incidents in which a gunman killed five or more people other than himself, which is notably a higher casualty count than is generally applied for tallying mass shootings in the U.S.dropped from 13 in the 18-year period before 1996 to zero after the Port Arthur massacre. Between 1995 and 2006, gun-related homicides and suicides in the country dropped by 59 percent and 65 percent, respectively, though these declines appear to have since leveled off. Two academics who have studied the impact of the reform initiative estimate that the gun-buyback program saves at least 200 lives each year, according to The New York Times."
How Australia Restricted Guns After a 1996 Massacre - The Atlantic