The founding fathers never imagined that freedom of expression would extend to the internet. Freedom of the press would extend to 24 hour news channels. Freedom of religion would allow for televangelists. Search and seizure protections would extend to the inside of a car or the hard drive of a computer.
The fact that they never imagined the technological advances we have made does not mean their ideas do not extend to protect them.
They did not have to.
You are under the impression that the Founding Fathers were dreaming up freedoms for us to follow.
Here is a little clue about our country that seems to never have been taught to you.
The Founding Fathers created a document that restricts GOVERNMENT, not the citizen. They feared a tyrannical GOVERNMENT, and therefore, places obstacles and restrictions on what that government can do.
It is not about them imagining that we'd have automatic weapons. It is about them knowing how abusive governments are, and stopping them from ever interfering.
The Second Amendment is a right we have, not given to us by government. It is the responsibility of government to ensure that we continue to enjoy it. Government exists to protect our rights, not give them to us.