The kids have changed, not the guns, you can or could get a lot of firepower when you wanted to, especially back then. The AR platform has been around since the 60's What has changed...? It is not the guns, it is the kids or society in general that has changed, back then if you had issues with another kid, you handled it on the playground..and kids went to the playground to burn off excess energy. Some of my fondest memories of elementary school was playing soccer with my classmates and we went back into class feeling good because we burned off some of that excess energy that all kids have and we also bonded with our classmates. Now there is no playground and to control the natural energy, they push medication. Also back in the day kids knew their roles in society, now you have several generations of boys that are told that "they are evil" and that all the ills of society are their fault, I hear the term "Toxic Masculinity" used so much. Boys have no hope, when you are told since elementary school that you are "evil" and responsible for all the bad things of society and that you have to atone for all the imagined slights and sins of your gender and treated as a potential rapist and especially if you are a white boy, you have the added sin of your race to "atone" for..Boys can't be boys, girls want to be boys and you don't know which bathroom to use. That is a hell of a burden to dump on a kid.
You also have parents that no longer parent, but want to be "the kids best friend", that isn't our role as parents...our job is to prepare the next generation to succeed and to be honorable productive members of society. When I was in school, if I screwed up, the teacher notified my parents and I caught hell, I make jokes about being a wooden spoon survivor but today the parents will support the kid no matter what against the teacher and this erodes the authority of the teacher and the principal.
Kids today have not been taught to respect authority and to avoid repercussions of their decisions. This is a corrosive effect on the kids mental well being. a kid has to have definite boundaries set, that is the job of the parents to instill the"Rules to live by" in a polite society.
You have boys being raised by single moms because the destruction of the nuclear family which has been proven through generations of struggle is the most stable for raising kids. Now you have single moms raising kids because the fathers left and a big reason is that there is no longer a stigma attached to fathering kids and booking. Also there is no stigma for moms having multiple kids through multiple fathers for "benefits". This all is destructive and corrosive to the kids mental well being, you have a generation of feral boys running around with no positive male role model influence for the sons.
Now people will still "blame guns" because it is easier to blame an inanimate object that has no soul and no morality and the soul or morality of the use depends on the makeup of the user. This goes where blaming an individual for doing a bad thing is not acceptable anymore, it is easier to blame "something" rather than "someone" for the evil in their heart.
I call it "no longer believing in God", call it what you will, but when people no longer believe in a higher power than the here and now is all that matters and the future no longer is a concern. We as a society have fallen from grace and no longer believe and the results are in front of us. We as a society glorify poor behavior, disrespect for each other, violence to each other and poor manners.
I don't know what it will take to change it, but screaming "Gun control" ain't going to fix it, the problems are much deeper and slapping a band-aid on the problem will not solve it. The kid, I will not use his name broke a slew of laws and adding another one will not solve the problem and stop the evil in a persons heart.