Another line of reasoning which makes the most sense to me is this: If the Framers of the Constitution and the Bill of Rights wanted the 2nd Amendment to specifically refer to a State's right, rather than an individual right, the wording of the Amendment would have been, "A well regulated Militia, being necessary to the security of a free State, the right of the MILITIA to keep and bear Arms, shall not be infringed.
But that's not what it says. It says, "the right of the PEOPLE to keep and bear Arms ..." Nowhere else in the Constitution, or the Bill of Rights, does the phrase, "the People," refer to the State. Look at Amendment 4. Interpreting "the People" to mean "the State" is ludicrous. Same with the 1st Amendment. Same with the Preamble.