Well that's kind of the key point of making some rules (and training) about handling firearms, to have a legally valid reason to prohibit the irresponsible peop

Well that's kind of the key point of making some rules (and training) about handling firearms, to have a legally valid reason to prohibit the irresponsible people from handling firearms if they have demonstrated that they won't, and can't use "I didn't know" as an excuse after they have decided to be stupid. The first step, of course, is coming to an agreement whether that subpopulation of 'irreversibly irresponsible' people should have the right to bear arms or not; in USA there doesn't seem to be a consensus about that.