To understand the Second Amendment and what the Founders who wrote it were trying to do, you have to understand the conceptual framework that they were working in. And that conceptual framework is based on two concepts regarding the military: the concept of the army and the concept of the militia.
The Second Amendment is about how the military power of the United States should be organized. It grew out of one of the most pretentious issues faced by the Philadelphia Convention, which was: what military power should they give this new Federal Government? They're sitting down to write the Constitution, and one of the three or four most important, most contentious issues they face is, should the Federal Government have an army?
Under the Articles of Confederation, there was no Federal Army. If the congress wanted to field armed forces under the Articles, they had to call on the states for requisitions. That was a model that some delegates to the Philadelphia Convention wanted to keep. They didn't want a Federal Army. Why? It starts with, as I say, the concept of army and militia. To the founders, these were two very different things.
David Yassky, Panelist (Symposium: The Second Amendment), 10 Seton Hall Const. L.J. 821 (2000), http://digitalcommons.pace.edu/lawfaculty/931/.