Well, to be completely correct, the 2nd amendment doesn't mention firearms, but only arms. When it was passed, private ownership of cannon and even cannon armed warships, the "ultimate weapon" of the day, was not unheard of. That can be seen by the Constitution's grant of power to Congress to issue "letters of Marque and Reprissal". No point in being able to grant a "hunting license" (to hunt foreign ships) if no one can own a gun (in this case cannon armed privateers). Individuals often owned cannon too, although ownership by a town or township was more common.