They don't.
Part I, Article XVII of A Declaration of the Rights of the Inhabitants of the Commonwealth of Massachusetts:
"The people have a right to keep and to bear arms for the common defence. And as, in time of peace, armies are dangerous to liberty, they ought not to be maintained without the consent of the legislature; and the military power shall always be held in an exact subordination to the civil authority, and be governed by it."
In Commonwealth v. Davis, 343 N.E. 2d 847, 849 (1976), the Massachusetts Supreme Judicial Court has held that the above does not guarantee individual ownership or possession of weapons. The supreme court reasoned that the right to “bear arms” contained in Pt. 1, Art. XVII concerned the custom of keeping arms for use in militia service and was “not directed to guaranteeing individual ownership or possession of weapons.”
Then the people of Massachusetts are screwed.
It's not like the electorate are tied down anyways. The fools continue to elect politicians who think the ideas of liberty and freedom are forbidden (Unless it concerns the "freedom" that gives women the right to murder the unborn).