"A well regulated Militia being necessary to the security of a free State, the right of the people to keep and bear Arms shall not be infringed."
- The Second Amendment to the Constitution
Note how it says: "the right of the people". Funny how smack dab in the middle of a Bill of Rights guaranteeing individuals protection from the overreaching hand of government, there are individuals who want to claim this is some right of the "states" rather than an individual right. Ironically, many who make this claim are dyed-in-the-wool federalists who would like to ignore the Tenth Amendment as often as possible!
As noted below, superficial fools who want to make the Second Amendment obsolete because of changes in firearm technology are not willing to apply the same restrictions to the First Amendment (nor should they) and restrict political speech to pamphleteering with hand cranked presses and cranks shaking hands in the public square! (Maybe the Keith Olbermann types would like to trade their teleprompters for quill pens?)
So, the argument "the Founding Fathers never envisioned semi-automatic weapons" holds as much water as "the Founding Fathers never envisioned television or high speed presses."
Which is to say, none at all.
What part of "shall not be infringed" don't you understand, K.O.?