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Second, and much more important, the Second Amendment (and the rest of the Bill of Rights as well) originally restricted only the federal government, not the state governments. There was little need for the Framers to be concerned about the details of the inevitable tradeoffs between individual freedom and public safety because the Constitution left the states free to balance those competing goals in whatever ways they thought fit. Every state was left free by the federal Bill of Rights to establish an official religion, to require a government license in order to publish a newspaper, or to abolish the right of trial by jury. Similarly, the states were left free to regulate the private possession of weapons in whatever way seemed appropriate to them. The Framers could therefore have reasonably expected that new issues, like those raised by technological developments in weaponry, could and would be addressed by the state governments as they arose.
This is a second argument. One that has been proven false time and again.
George Mason said it best to debunk this one:
There is no Declaration of Rights, and the laws of the general government being paramount to the laws and constitution of the several States, the Declarations of Rights in the separate States are no security. Nor are the people secured even in the enjoyment of the benefit of the common law.
Since George is given credit for penning the Second, we'll go with his interpretation of what he meant:
"I ask, sir, what is the militia? It is the whole people, except for a few public officials."
George Mason, in Debates in Virginia Convention on Ratification of the Constitution, Elliot, Vol. 3, June 16, 1788
Then again, Albert Gallatin said it well too:
"The whole of the Bill (of Rights) is a declaration of the right of the people at large or considered as individuals .... It establishes some rights of the individual as unalienable and which consequently, no majority has a right to deprive them of." -- Albert Gallatin, New York Historical Society, October 7, 1789