Related: UA Criminology Professor: US is # ! in mass killings

His study indicates that 31% of the mass murders on earth between 1966 and 2013 were committed in the U.S.
     And yes, gun ownership seems to be behind it. Newsweek reports:

With data for 171 countries, Lankford found that the United States had by far the most public mass shooters, with 90 during the 46-year period. That’s five times as many as the next country on the list—the Philippines, with 18. Rounding out the top five were Russia (15), Yemen (11) and France (10). In other words, although the U.S. accounts for less than five percent of the world’s population, it had 31 percent of mass shootings between 1966 and 2012.