Here is a list of mass murders from 1900 till 1999. Semi auto rifles have been available since 1903. Yes, 1903.
Yet the gun deaths do not really increase until the 1970s, after the mental hospitals were closed and the inmates dumped on the streets.
https://en.wikipedia.org/wiki/List_of_mass_shootings_in_the_United_States_(1900%E2%80%931999)
Now see how TIME tries to hype the mass murders from 1999 to 2021..
https://time.com/4965022/deadliest-mass-shooting-us-history/
I should also mention that these links are only gun crimes. Richard Speck who murdered 8 student nurses in 1966 using a knife is not even mentioned.
He is finally mentioned in ...
https://en.wikipedia.org/wiki/1966_Rose-Mar_College_of_Beauty_shooting
Perpetrator
The gunman, 18-year-old Robert Benjamin Smith, a resident of Mesa, surrendered without incident to responding police. Smith was a high school senior at the time of the attack.
Smith told police that he was inspired by mass murderers Charles Whitman and Richard Speck who had carried out mass murders earlier the same year. Smith told responding police that he simply sought infamy and wanted to be known and remembered. Smith explained that he had hoped to kill ten times as many people as he had. It was later observed that Smith had become captivated with historical figures such as Napoleon Bonaparte, Julius Caesar, and later Adolf Hitler. After the assassination of John F. Kennedy in 1963, Smith also became awe-struck by Kennedy’s assassin, Lee Harvey Oswald.[2]
He was considered to have had mental health issues. As of 2023, he is still alive and in prison,