How common are mass shootings in the U.S.?

<p>The U.S. has had more mass shootings than days in 2015.&nbsp;<a href="http://shootingtracker.com/wiki/Mass_Shootings_in_2015" target="_blank">352 shootings in 336 days</a>. (Video via&nbsp;<a href="http://www.nbcnews.com/" target="_blank">NBC</a>)&nbsp;</p><p>That&#39;s from&nbsp;<a href="http://shootingtracker.com/" target="_blank">ShootingTracker</a>, a group that tracks and reports on<i>&nbsp;</i>gun violence. (Video via&nbsp;<a href="https://www.youtube.com/watch?v=Z656XtgKhCE" target="_blank">KMGH</a>)&nbsp;</p><p>It defines &quot;mass shooting&quot; as four or more people injured with a gun. That includes the gunman. (Video via&nbsp;<a href="https://www.youtube.com/watch?v=A9JuWhM1doU" target="_blank">ABC</a>)&nbsp;</p><p>ShootingTracker says 2014 had&nbsp;<a href="http://shootingtracker.com/wiki/Mass_Shootings_in_2014" target="_blank">336 mass shootings</a>. That&#39;s for the entire year. (Video via&nbsp;<a href="http://www.msnbc.com/the-ed-show/watch/marysville-police-give-press-conference-on-shooting-347557443809" target="_blank">MSNBC</a>)&nbsp;</p><p><i>This video includes music from&nbsp;</i><a href="http://freemusicarchive.org/music/Chris_Zabriskie/Divider/02_-_Candlepower" target="_blank"><i>Chris Zabriskie / CC BY 4.0.</i></a><i>&nbsp;</i></p>