The problem is, in most cases, they are either pushing for redundant laws (duplication of laws that already exist), laws that violate existing privacy laws (even if someone has seen a doctor for mental health issues, that is by law, private. Also even if it wasn't, you would actually create a dis-incentive to seek help by linking it to gun rights), or violate due process (you cannot arbitrarily strip someone of constitutional rights unless they have, at the very least, been charged with a crime. Being on a "watch list" is not the same), also any laws must avoid having a "discriminatory impact" (I.e. any law that disproportionately affects blacks, Muslims, or any other minority would not be allowed).