Millions of M1 Carbines were widely and affordably available for years before the AR-15 was a thing. It, like the AR-15, is also an easily-handled magazine-fed semi-automatic rifle firing intermediate cartridges, and was intended for military service.
Virtually no school shootings occurred until Columbine (and the media coverage surrounding it, and the miserable state of American society) set off the waves of shootings that continues to this day. It’s worth noting they didn’t use either AR-15s or M1 Carbines, that didn’t become common until later.
If the AR-15 is the cause of this because it is an easily-handled magazine-fed semi-automatic rifle firing intermediate cartridges, how do people explain the near complete lack of mass shootings despite the wide adoption of the M1 Carbine in a time when gun ownership was even less restrictive?
Not a hard enough question? Ok try this one: actual machine guns used to be widely available and much more affordable than they are today. Why is there relatively little recorded violence with them?