At the time of the ratification, "arms" basically meant a weapon that could be carried by a soldier. The next larger class of weapons would be "ordnance", to in

At the time of the ratification, "arms" basically meant a weapon that could be carried by a soldier. The next larger class of weapons would be "ordnance", to include things like cannon and explosives. It's also worth noting that the Constitution implicitly recognizes the civilian ownership of ordnance as well, through Article 1 Section 8: "To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water" granting letters of marque wouldn't make sense if individuals did not possess suitably armed ships.