http://www.attn.com/stories/2893/america-mass-shooting-problem
Given the fact that the United States has over 200 million more firearms in circulation than any other country, it's not surprising that our public mass shooters would be more likely to arm themselves with multiple weapons than foreign offenders
If people want to reduce the number of mass shootings in the U.S., a good first step would be reducing the number of guns in circulation.
