Keeping semi-automatic weapons out of the hands of those with mental problems sounds wonderful.  Of course, the devil is in the details.  We have no way of accomplishing this feat without disenfranchising many many people who do not realistically fall in the category of unstables.  To ban gun ownership of all who have sought mental health services will capture so many persons who are not a threat that we have a problem.  Even those seeking counseling for abuse would have a mental health record.  If we say that mental health workers will identify only the bad guys, they will be able to arbitrarily disenfranchise any individual they do not like - without a trial or conviction or without review.  Just one dude making a report can disenfranchise a citizen.   The terrorist list?  Who puts somebody on it?  How does one get off?  Is it reasonable to disenfranchise someone who has never been allowed to tell their side of the story?