Weirfan
I understand where you're coming from on the mental health issues. The problem with statistics is they don't take into account the number of people who have issues that don't interfere with their day to day lives so they're never diagnosed. How many of the mass killers were perfectly normal, law abiding citizens until something in them snapped. Maybe they had some sort of head trauma that rewired their brains. It could even be an infection that has made its way to their neurological system. I know it sounds like I'm going out on a limb. I guess my point is there is no one or two issues that cause this.
As someone who owns a couple semi auto rifles I will say that I don't believe banning them will do much, if anything. I also own a few lever guns/cowboy guns. Technology from the mid-1800's. I can fire just as fast and as accurately with those as I can my seniautos. The only thing I lose is capacity. I read that he had 13 firearms in his room. A skilled marksman could easily take out more than 50 people with 13 lever guns.