After a police officer's 12-year-old son got access to the officer's handgun, the officer was prosecuted for violating Mass. Gen. Laws. ch. 140, § 131L:

It shall be unlawful to store or keep any firearm, rifle or shotgun ... in any place unless such weapon is secured in a locked container or equipped with a tamper-resistant mechanical lock or other safety device, properly engaged so as to render such weapon inoperable by any person other than the owner or other lawfully authorized user. For purposes of this section, such weapon shall not be deemed stored or kept if carried by or under the control of the owner or other lawfully authorized user.

Last month, the court held the statute was unconstitutional (Commonwealth v. Bolduc), and dismissed the prosecution. I only just now managed to get a copy of the opinion, and here's the relevant discussion:

The locking mechanisms [required by the statute] are the functional equivalent of those enumerated in the D.C. statute struck down in Heller.

In Heller, the Court held that the Second Amendment not only protects an individual's right to possess firearms but that the right requires that the firearms be available for "the purpose of immediate self-defense." The Massachusetts statute mandating lock boxes or similar devices would frustrate an owner's ability to immediately access an operable weapon.

Although the statute exempts firearms that are "carried" or "under the control of the owner" from the requirement that they be locked, the statute applies to the lawful owner of a firearm even when he is at home. People can be subject to prosecution whether they are home or not. The term "under the control of the owner" is a question of fact and subject to interpretation. Any ambiguity in the statute as applied to a person lawfully keeping a firearm in the home must be resolved in favor of the holder of the right. Legislation requiring an owner to store firearms in a place inaccessible to children or unauthorized persons would satisfy the Supreme Court's holding in Heller and protect the safety of others.

In light of the foregoing, the Court finds that, based on the Supreme Court's decision in District of Columbia v. Heller, G.L.c. 140, sec. 131L is unconstitutional.

According to a Massachusetts Lawyers Weekly article notes that Massachusetts courts seem split on this. It also reports that the prosecutor "agreed with [Judge] Lynch's analysis and decided not to appeal. 'I've read the Heller case,' he says. 'Judge Lynch read the Heller case, and the Heller case seems to say very clearly that these kinds of blanket restrictions are unconstitutional.'"

Interestingly, the court seemed to assume that the Second Amendment applies to state laws -- what lawyers call the "incorporation" issue -- which is something Heller pointedly declined to resolve.