It's not registration of firearm possession, but of firearm transfers. Since most residents who have firearms obtain them while they're Massachusetts residents, it's pretty close to a registration of possession. The incompetence with which the system was previously handled means that there are huge gaps in the record. Since the only actual requirement is that firearm transfers be registered, one can move to Massachusetts with firearms and never be required to register them, contrary to what some police, dealers and other self-appointed experts may assert.
Ken