You not be institutionalized and have not been for five years would be a place to start.
The problem with the anti-gun crowd is not in finding a place to start. The problem is that they will never find a place to stop.
Whatever criteria one imposes on those who may be a risk, there will always be those who don't meet the criteria and yet commit the offenses. Additionally, a lot of people will be disarmed who, in fact, won't harm anyone.
Those who would limit the size of detachable magazines were satisfied for a while with 15 cartridges in New Jersey. We're down to 10 in Kalifornia. New York has decided that the magic number is 7.
There is a price we must be willing to pay to preserve our liberty. There is also a price we must be willing to pay if we are to continue eliminating morality and responsibility. It is perhaps unclear that those two issues can be disconnected.