There's only one policy of any kind that has ever been shown to deter mass murder: concealed-carry laws. In a comprehensive study of all public, multiple-shooting incidents in America between 1977 and 1999, the highly regarded economists John Lott and Bill Landes found that concealed-carry laws were the only laws that had any beneficial effect.
And the effect was not small. States that allowed citizens to carry concealed handguns reduced multiple-shooting attacks by 60 percent and reduced the death and injury from these attacks by nearly 80 percent.