I now want to point out that in fact, while the empirical evidence on the relationship between gun control and homicide is (at this time at least) utterly inconclusive, there certainly are policies out there that we have very solid evidence to believe would reduce gun-related homicides very substantially.
The one at the top of the list, in my view, is to legalize recreational drugs such as marijuana and cocaine.
The theory behind this policy prescription is that illegal markets breed competition-driven violence among suppliers by offering the prospect of monopoly profits and by denying them lawful means for enforcing commercial obligations.
The evidence is ample. In addition to empirical studies of drug-law enforcement and crime rates, it includes the marked increase in homicide rates that attended alcohol prohibition and the subsequent, dramatic deline of it after repeal of the 18th Amendment.