Some opponents of gun control, including NRA Executive Vice President Wayne LaPierre, say, "The only thing that stops a bad guy with a gun is a good guy with a gun" [source: Lichtblau and Rich]. But at least one study has shown that defensive gun use succeeds only rarely, and that gun owners are 4.5 times more likely to be shot during an assault [source: Branas, et al.]
It's hard to say how many mass shootings might have been prevented with tighter gun control laws. Because of the gun show loophole, the woman who bought three of the four guns used by the Columbine killers was never prosecuted, despite making a "straw purchase" for two minors [source: Violence Policy Center]. 2007 Virginia Tech massacre gunman Seung-Hui Cho passed a background check to purchase his weapons, even after a state judge had declared him mentally ill [source: Schmidt and Savage].
But work by University of Massachusetts researcher Louis Klarevas suggests that revival of the federal assault weapons ban might reduce the number of mass killings. For his 2016 book "Rampage Nation," Klarevas gathered data on every mass shooting incident in which six or more people were killed between 1966 and 2016. He found that compared to the 10-year period before the 1994 ban took effect, the number of such massacres dropped by 37 percent, and the number of fatalities decreased by 43 percent. But after the ban expired in 2004, the number of mass killing incidents increased by a startling 183 percent, and fatalities jumped by 239 percent [source: Ingraham].
But to really limit mass shootings, some say that it also will be necessary to find effective ways to identify and treat the mentally ill, and to limit their access to sensitive areas and weapons. Proponents of closer screening have complained that states aren't always diligent in submitting mental health records to the federal background check database, despite a 2007 federal law requiring it. They can point to the example of Jared Lee Loughner, who killed six people and wounded 18 others, including then-U.S. Rep. Gabrielle Giffords, D-AZ, in 2011. Though Loughner was suspended from a community college because of mental health problems, he still managed to pass a federal background check and purchase a weapon. The Brady Campaign to Prevent Gun Violence, a gun control group, estimated at the time that millions of mental health records were missing [source: Feldmann]. Preventing violent acts like this will require understanding and defusing the pathology that drives these killers -- a feat far easier said than done.