Admittedly I don't know anything about the 308 round. I have seen a 9mm version. Seems the 9 would start getting a tad heavy (??) when loaded and pretty ineffective due to the round. I actually have considered buying the S&W M&P 15 for about a year now. Really cheap at gun shows. The 22LR version is REALLY cheap. I would buy the 223 version though simply because I would only use the gun to defend my home if it ever came to that and that is an effective round for that purpose.
Hey I grew up in WV where it seemed every little kid in my town had a 410 shotgun and a 22 rifle for hunting. Just like everywhere else, these same kids brought their bigger hunting guns to school in the back of their trucks. Nobody shot up the schools. I completely understand that. This is my opinion; the AR style guns with the 223 round is a weapon used to terrorize by idiots wanting to terrorize. The platform is scary and therefore adds to the terror. The round is more lethal than your 9mm or maybe even a 40. Let's not forget the VT killed 31 with a 9mm and a 22 semi-automatic pistol. Those two worked well.
And to your point, there is no solution to the problem until we force parents to socialize their kids like puppies, quit "glorifying" the shooters, somehow be willing to trample on somebody's civil right because they might be crazy. And none of that might work anyway. Most everyone of these shooters had some sort if mental issue. However, look at the Las Vegas shooter, he likely wouldn't have been deemed incompetent by a court. Nobody expected him to kill over 50.
So yeah if raising the age to purchase to 21, imposing a realistic waiting period, and mandating background checks is a feel good measure (which I think age and waiting periods are for the most part), I still support it. We can't continue to do nothing here. Right? If I decide to buy that M&P 15 today, there is no reason in the world why I can't wait a week to pick it up. I just passed the background check. And I am well over 21 with a long history of responsible gun ownership. I don't believe that the ideas floated by Trump last week trampled on anyone's ownership rights. They would only serve as a potential impediment to that loony-tune going off on the public.