A question about Caetano v. Massachusetts

Yesterday, the Supreme Court issued a two-page unsigned opinion resolving a dispute between the Commonwealth of Massachussets and Jaime Caetano, who had been prosecuted for possession of a stun gun. The Court ruled that the law prohibiting stun gun possession violated the second  amendment.

In the brief discussion, the Supreme Court dismissed an argument made by Massachusetts. Massachusetts had argued that stun guns could be prohibited under an old common-law rule allowing prohibition of items which are “dangerous per se at common law and unusual”.

That’s a fine, and reasonable rule; there’s a difference between weapons (which are protected by the second amendment) and random dangerous things (such as, say, containers of ricin, or suitcase nukes).

But where exactly does that line lie? The Supreme Court didn’t get into it, because Massachusetts had conflated “unusual” with “in common use at the time of the Second Amendment’s enactment”, a standard which result in a second amendment that functioned as the first amendment would function if it were held not to apply to the internet because the authors of the first amendment couldn’t have imagined computers.

Yet it’s an interesting and important question. What’s the line between an unprotected dangerous per se object and a protected weapon?