In what I call "firearm psychology", people who wish to obtain a gun would have to schedule a session with a licensed arms psychologist and submit to a full MRI scan of their brain. This would allow us to see who is at risk for committing violent acts and therefore should not be sold a firearm. Lesions to the frontal lobe (the part of the brain associated with judgement, self control, planning) can cause people to become violent. In a study done of 70 notorious serial killers, ALL 70 had frontal lobe damage caused in their childhood by accident or by abuse.
http://socyberty.com/crime/a-killer-brain-a-look-inside-serial-killers/
We now have technology to see who is at risk of becoming a psychopath and committing violent acts. All we have to do is apply it.