From Boston Globe:

So what makes this state’s gun laws effective? Two elements are key: All gun sales, even those from private sellers, incorporate a federal background check at some stage in the process. Secondly, to purchase a gun, someone must first apply to the local police chief for a renewable license; even if the applicant passes the federal background check, the chief can deny a license if, based on reliable information, he or she deems the applicant a risk to public safety or to themselves.