Here's the Wikipedia list of the 20 deadliest shootings in America since ~1950 (actually 23 total shootings). From the data, it looks like these are all shootings since ~1950 where ten or more people, including the perpetrator, were killed.
Of those listed above, the AR-15 rifle was only used in nine of them. Handguns were used in 14 of the shootings. Semi-auto rifles
other than the AR-15 were used in five of them. Bolt action rifles were used in two. Shotguns were used in seven.
The deadliest school shooting in the US featured a Glock 19 and a Walther P22.
Of the 23, seven occurred at schools - two at a university, one at a community college, one at an elementary school, and three at a high school.
Of the 23, 14 occurred this century and 16 in the last 27 years (since the Luby's massacre in 1991). Of those 16, only
3 5 were committed by perpetrators born before 1980. In other words, if you take 1980 as the starting birth year for the Millennial generation,
13 11 of the deadliest shootings in America were committed by "Millennials".
Looks like the perpetrators mostly suffer from mental health issues or were terrorists, or both.
Edit: I miscounted. 11 of the shootings were committed by "Millennials". However, of those 11 shootings, there were 13 perpetrators.
