I've seen this proposed a lot and it's frustrating because it sounds like it's a reasonable idea. Unfortunately, it's not and passing a requirement for it would make a lot of things a lot worse.

For context, I've worked in mental health now for ~15 years. I'm not a clinician and currently I work in youth mental health.

For starters, I want to make it clear that the connection between mental health and violence is based on a number of things but one of those things is not reality. For starters, people with some form of mental illness are much more likely to be the victims of violence than the perpetrators of violence. From professional experience, many forms of mental illness often cause some form of social impairment that can make conflict more likely or make someone more susceptible to being perceived as an easy target for violence. The conversation about violence and mental health is weighted far too heavily in the opposite direction from reality.

We might say "Well shooting up a school is a violent act that no sane person could consider doing." The very uncomfortable truth of the matter is that you absolutely do not have to be mentally ill to consider doing this. There was a ton of psychological research done on former Nazis after WWII with the conviction that they must have all be heinous psychopaths* because who could possibly consider taking part in something like the Holocaust normal? They found that the majority of former Nazi officials and even soldiers and camp guards had no greater proclivities towards sociopathy or anti-social behavior than the average person. We also have to consider in that same vein that we have historically ordered soldiers to do pretty horrifying things in the past and their willingness to do them was not considered a mark of some sort of depravity. The crews of the bombers that dropped the first atomic bombs were not shunned as mass murderers for killing tens of thousands of civilians.

The long and the short of it is that you don't have to be mentally ill to carry out acts of unspeakable cruelty and violence. Having a fucked up moral compass isn't a mental illness and such acts of violence, if framed properly and given proper social context, can be lauded as positive things.

With that in mind, it's important to understand that there's no real way to tell if someone is going to pose a threat, either to others or themselves. Clinicians aren't psychics and while you may get a sense that someone intends some harmful act, there's no definitive way to tell short of someone telling you "I want to go out and hurt people/myself."

I think people imagine there's some kind of Voight-Kampff test that a clinician can do that will pick out a mass shooter when that's not really possible.

For any diagnosis or meaningful determination to be made in a clinical setting, a clinician often has to develop some kind of rapport or understanding of the person they're helping before making an assessment. This often takes time, depending how responsive the person is to the clinician. Different people respond different ways to different things and it's impossible to have a broad spectrum "checklist" to determine if someone poses a threat or not. Someone talking about suicide may be doing so as a joke, a way to deflect personal feelings or they may be openly discussing plans of harming themselves. There's no way to tell unless you've spent time with this person and have some familiarity with their overall framing.

It's also important to remember that there is no "sanity test." Part of what makes something a mental illness is an element of distress and disruption a particular behavior brings to someone's life. Someone who feels compelled to clean often and feels mildly uncomfortable if they can't is not the same as someone who has thoughts of needing to clean their environment such that they stay awake for hours or miss work to scrub and clean because they feel compelled to do so. If someone feels compelled to clean for hours but is able to do so in such a way as to not disrupt leading an otherwise healthy life or feel distress, they may still qualify for a diagnosis of some form of mental illness but the lack of distress or disruption means it's hard to say this person has a genuine mental illness.

Remember, someone who does something that you don't understand the motivations for is not necessarily mentally ill. They are not insane because you don't understand why they do what they do.

Even if a clinician puts in the time with a person, the best they can do is determine that that person isn't currently a threat to themselves or others. They can't see into the future and see where life changes might take a person's mental health. Someone who has problems but leads a happy, healthy life with medication to help manage symptoms may, for whatever reason, have access to medication disrupted despite having no intention to discontinue their use or may suffer a worsening of symptoms due to an adverse life event or even just advancing age. Some elements of certain forms of mental illness often don't manifest until certain points in a person's life so if someone undergoes an evaluation at 20, they may be fine until 35 when some hereditary form of mental illness starts to kick in.

Asking a clinician to put a "not dangerous" stamp on someone before they can buy a gun also raises the specter of liability. If a clinician green lights someone and they go out and kill people the next week, do we consider that green light to have been wrong? What if they do it six months later? A year later? If a clinician who has cleared hundreds of people loses their license, do we have to re-test all the people they green lit? Can someone who was denied sue if they feel the decision wasn't reached in a proper manner?

If I'm a clinician who is very pro or very anti-gun, can I just rubber stamp approve/deny everyone to conform to my own beliefs on the subject? You could argue that that's losing my obligation to professional objectivity but I could argue either way that (pro) I'm being asked to do something I can't in good conscience do so I'm going to rely on people's judgement or (anti) that it's a public health problem to allow people access to firearms and that I'm abiding by my professional responsibility to clients to keep firearms out of their hands. Either way, I have a solid argument for the ethics board.

These are all incredibly thorny issues that, while not impossible to solve, have no solutions that don't open the door to a myriad of other problems. If I were a clinician, that all sounds like a nightmare and I'd frankly just refuse to take anyone for an evaluation. It's too much of a risk from my standpoint.

Even if we could somehow legislate a solution to that, there's the problem of who is going to pay for this? A mental health professional's time is not free and that cost has to come from somewhere. Ultimately it's going to mean wealthy people who can afford it are going to have no problems getting what they want and poor people aren't going to be able to afford to see the person they'd need to see to get cleared. This gets even worse if you want to make this a yearly exam. Like most gun control policies, rich people get effectively a free pass.

Likely the worst outcome is you are disincentivizing people who own firearms from seeking out mental health treatment. If you own guns, you may feel hesitant to see a therapist now because what if what you want to talk to them about makes you fail your psych authorization for a purchase? If you're struggling, you may avoid seeking out help out of fear that you'll be flagged. The last thing we need to be doing is motivating people to not seek out help when they need it.

Again, I know this sounds like a good idea but the reality of the situation is it's not workable and it's not going to produce the results that people seem to believe that it will. It's unlikely to catch anyone planning acts of serious violence and it's going to keep people away from resources they may desperately need.

Please, stop pushing this as an idea.