A pretty simple how for that case would be to have a protected database where mental health professionals and institutions would report individuals with issues deemed worryng enough to bar from purchasing a gun. Then during the background check they would reference that db. If the person being checked is verified to be in that db fail the check. Maybe have some revaluation options or whatever but it’s not hard to imagine how reasonable laws that are actually enforced could actually help. The half baked laws that are half assed enforced and then held up as an example of any laws at all being fundamentally impossible just isn’t convincing.