Maybe the problem is the underlying premise that something can always be "done."
A mentally-ill person had unusual access to semi-automatic weapons, and was trained in their use, and tragedy resulted.
Sometimes a truck will blow a tire and skid into a school bus. Shit happens. You can't really make a workable world where things like that are impossible, or even much more improbable.
The belief that everything can be cured if you just make enough laws is not borne out by history.
That, and you will always be left with those who don't obey the rules anyway.