Update (April 17, 2018): The Massachusetts high court ultimately decided, in another case, that the state's absolute bar against civilians possessing or carrying stun guns violated the Second Amendment. The court determined that "the possession of stun guns may be regulated, but not absolutely banned." (Ramirez v. Commonwealth, No. SJC-12340 (Mass. 2018).)
The Supreme Court of the United States has decided that the Second Amendment right to bear arms applies to stun guns. In so doing, the Court calls into question a Massachusetts state law outlawing the devices. (Caetano v. Massachusetts, 577 U.S. ___ (2016).)
The Supreme Judicial Court of Massachusetts had upheld a state law making the possession of stun guns illegal; it ruled that the Second Amendment doesn’t apply to stun guns. The U.S. Supreme Court, in a per curiam decision, overturned that state-court decision.
The U.S. Supreme Court quickly tossed aside the Massachusetts court’s reasoning. It noted that the Second Amendment presumptively applies “to all instruments that constitute bearable arms, even those that were not in existence at the time of the founding[.]” The Court cited its Heller decision for the proposition that the Second Amendment extends to weapons that didn’t exist in the 18th century. It also rejected the notion that the Second Amendment protects only “those weapons useful in warfare.”
The high court, however, stopped short of declaring laws that ban stun guns unconstitutional. Instead, it faulted the explanation the Massachusetts court gave for its ruling and basically told that court to reconsider the case in question.