I've been pondering the mentally ill question for several days. I think we can all agree that, at least in theory, people who are mentally ill should not be allowed to own firearms. Scalia even acknowledges this in Heller. But how to categorize who is mentally ill without violating privacy rights? Of course we could limit this to people in mental institutions (which, as Joe Bryant pointed out in the OP, the law already does) but we all know that most people who are mentally ill don't ever enter into an institution. We could revamp Obama's idea of making a list of those who receive medication for mental illness from the government, but beyond this still be very limiting (it excludes all of the people who get the same medication from private sources), the ACLU objected, again for privacy reasons. And though I disagree with them on this, I'm troubled by their point. The vast majority of mentally ill people don't commit crimes and shouldn't be treated as if they do.
One way to start, which I have mentioned before, is to extend gun prohibitions to those who commit violent misdemeanors. From what I've read this is almost a standard characteristic of these mass shooters- at one time in the past they committed an act of domestic violence, but because it was not a felony, it didn't stop them from gun purchases later on. It should. Another idea would be to allow juvenile crimes to play a role in prohibiting gun sales. Juvie records are typically sealed and nobody's allowed to look at them, but I think that if you were convicted of committing a violent crime at any point in your life, that should prevent you from being able to legally purchase guns for the rest of your life.
Anyone have a problem with this?