The San Bernardino shooting is the 355th mass shooting this year, according to a mass shooting tracker maintained by the Guns Are Cool subreddit. The Reddit tracker defines mass shootings as incidents in which four or more people, including the gunman, are killed or injured by gunfire.
The Mass Shooting Tracker is different from other shooting databases in that it uses a broader definition of mass shooting — the old FBI definition focused on four or more people killed as part of a single shooting.
Speaking after the Colorado Springs shooting last week, President Obama urged Americans to not let this type of violence "become normal." But the data show that this type of incident already is normal. There have been more mass shootings than calendar days so far this year.