Actually that is not true. Unless there has been a ruling that because of your mental state you are not allowed to own a gun, there is nothing stopping someone from buying one. Any violent criminal history involving a gun also will keep you from being able to buy a gun (as it should be). That background check they do is to make sure that you are not a felon and that you do not have a history of violent crimes even if the conviction was only on a misdemeanor charge.