"High-Capacity Magazines
High-capacity magazines — or assault weapons likely equipped with them — were used in at least 15 of the incidents (11%). These incidents resulted in an average of 13.3 total people shot — 155% more people shot than in other incidents (5.2) — and 7.5 deaths — 47% more deaths than in other incidents (5.1)."
"Mental Health
Of 133 examined incidents, in only one was there evidence the shooter was prohibited by federal law from possessing guns due to severe mental illness. In 15 other incidents (11 percent), there was there was evidence that concerns about the mental health of the shooter had been brought to the attention of a medical practitioner, school official, or legal authority prior to the shooting."
"Role of Prohibited Possessors
Certain categories of people, including felons, certain domestic abusers, and severely mentally ill people are prohibited by federal law from possessing guns. There was sufficient evidence to judge whether the shooter was a prohibited gun possessor in 116 of the 133 incidents (87%). Of those 116 incidents, 44 (38%) involved a prohibited possessor, and 73 (62%) did not."
"Workplace and School Shootings
Five of the 133 shootings (4%) occurred at the shooter’s current or former workplace. Five of the 133 shooting incidents (4%) took place in schools, including primary, secondary, and college campuses."
"“Gun-free” Zones
Ninety-one of the 133 incidents (68%) took place wholly in private residences. Of the 37 incidents in public spaces, at least 21 took place wholly or in part where concealed guns could be lawfully carried. All told, no more than 17 of the shootings (13%) took place entirely in public spaces that were so-called “gun-free zones.”"
http://everytownresearch.org/reports/mass-shootings-analysis/