The problem isn't mental health, or any model of firearm.
The problem is... the people themselves.
Within my own lifetime, I've seen Americans go from a people who wanted to live free and accept the responsibility for that decision, and understood there were consequences if you made poor decisions, to a bunch of whining sheep who want Daddy Government to take care of them and who refuse to lift so much as a finger for themselves.
The AR-15 isn't a new technology. It has existed since the 1950's. Semi-automatic, magazine-fed firearms have existed for over a century. There was a time when guns were accepted, everyday things; pretty much everybody had a small .32 or .38 they carried as habitually as people today carry smartphones. Kids were exposed to firearms and trained in their use from the time they could walk, and regularly took guns to school. Not for anything as benign as Show-N-Tell but because the kids would be using their gun to take small game for the family stewpot on their journey home after classes! And you NEVER had mass shootings or school shootings!
Sadly, the people now don't want to be free. They don't want to take responsibility for themselves. They want to be taken care of and protected so they can "feel safe". It's disgusting, and it's pathetic.