In the late 70's Carter's administration and Congress passed an act that put the responsibility of mental health on the states. During that time there was hyperinflation, and many of those states adopted stricter fiscal policy. Funding for mental health was put under the label of social programs, and received a lot of cuts.
A decade earlier, mental health professionals were debating mental health asylums, and decided that most cases were minor and unnecessary. They released a report calling for mass integration of former patients with minor psychiatric problems into the general public.
So today, for someone to be committed against their will usually require a psychologist's opinion; in most cases as a danger to others or themselves. Even so, most will only be held for 72 hours.
More information:
http://www.sociology.org/content/vol003.004/thomas.html
I know this discussion is about gun control, so I apologize for talking only about mental health, but there were a few people who posted questions that I felt I could answer. I used the above source to make sure I was correct.
Personally, I think you could lock up every single person that shows a hint of mental disorder and still have people pushed to their limit, and blaze up a school or public place. Everyone has a pre-disposition or stress level. When that level is breached (extreme stress) our minds become dysfunctional.
I don't think we truly understand what new technology does to the human psyche. The field of Psychology is still very much in the 1990's. We are more connected than ever before. All forms of tragedy are broadcasted on Facebook, Twitter, etc. All of this new information, I believe, adds new levels of stress that we previously never encountered.