Yeah that's where it gets tricky: there's plenty of people who have mental issues, breakdowns, etc, who get over it and are perfectly fine afterwards. Given that 2A is a Constitutional right, it's dangerous to haphazardly put in laws that would restrict those rights for large groups of citizens. Maybe a guy is in school, he's stressing over SATs, his girlfriend leaves him, his mom dies, and he has a mental breakdown and has to go to the doctor. So he's now restricted his 2A for the rest of his life? Doesn't make much sense to me. And of course there are so many people with mental illness who would never dream of hurting other people.
As far as the AR goes... Seung-Hui Choi killed 32 people at Virginia Tech with just two handguns, so an AR ban is not the magic quick fix people think imo. This recent kid used a shotgun and handgun I think.