I want to bring up this point again, since all of the solutions proposed tackle the symptoms of the issue, not the issue itself. I've said this a few times in different threads here, but when you're looking to fix a problem, you're actions need to be directed at what causes the problem. A good way of thinking about it is that you shouldn't attack the symptoms of a disease, you should attack the disease itself.

In mass shootings, mental illness and antisocial behaviour is what's causing the problem. Actions should be directed at tackling those issues.