The Framers recognized that self-government requires the People's access to bullets as well as ballots. The armed citizenry (militia) was expected to protect against not only foreign enemies, but also a potentially tyrannical federal government. In short, the right to bear arms was intended to ensure that our government remained in the hands of the People.
Akil Reed Amar and Alan Hirsch, For the People: What the Constitution Really Says About Your Rights, (N.Y.: Simon & Schuster, Inc., 1998).
Foreign enemies aren't a concern anymore: your government takes care of it for you.
As for a tyrannous government, you're outgunned, plain and simple: if no other country in the world can match the US military, what hope have citizens? Like I said, if you want to overthrow a tyrannous government, you'll need artillery, not semi-automatic or automatic weapons. And your government won't let you do that.
