Won’t work. Criminals ignore laws, even new ones.
Of course, it will work.
Gun Control is not about preventing crimes. It is about controlling people. Specifically, it is about disarming law-abiding peasants such that they cannot even imagine resisting orders from the Rulers.
Criminals are allies of the Rulers who are used to keep the population terrorized and submissive.
Criminals will always be able to obtain firearms, and as long as they confine their predations to the "right" people, they will be allowed to continue them.
It will work just fine. It’s not about stopping mass shootings. It’s about slicing away the right of self defense one law at a time.