The Second Amendment of the US Constitution is the only part of our founding document that says why it exists. Sure, the beginning of the Constitution says that the government should "establish Justice, insure domestic Tranquility, provide for the common defence, and promote the general Welfare*", but it doesn't say which part of the Constitution provides for which part of this mission statement. Is the commerce clause there to "promote the general welfare"? That's how it's taken nowadays, but probably not originally.
This question of "The Founders' Intent"** is a persistent one. People see all sorts of intent throughout the Constitution, but it generally differs based on the person's preconceived notions of what's right and wrong. The Second Amendment is the only exception to this rule: "the right of the people to keep and bear Arms, shall not be infringed", because "A well regulated Militia, [is] necessary to the security of a free State". Now, the Constitution Does Not say that the right to bear arms is limited to running a militia, so the extremists are closer to correct than the anti-gun folks. I say closer to correct because even the pro-gun extremists generally don't think a person should be able to own a battery of Howitzers, or helicopter gunships, or a private battleship patrolling the Erie Canal. Unfortunately, this is exactly what the Constitution doesn't just allow, it's what the Constitution demands.
The Constitution boldly says that a militia is necessary for freedom. This is an extreme position, there's no mistaking it. They don't mean some sort of Ren Fest organization with muskets or swords, they mean a modern fighting force. There's no point in having a militia that can't fight. Having a fighting military today means heavy weaponry on a scale that the founders could not have imagined. Three guys in a jeep have about as much firepower today as the largest gunships in 1776, if not more. If you concede that the United States is at least relatively free, or that it has been in the past 50 years, then you must conclude that the Second Amendment is factually wrong, since we haven't had a militia in the 1776 sense for at least that time. An intellectually honest people would take the honest approach and have a Constitutional Convention. We'd hash it out in the public forum, and we'd probably make a mess of it, but it probably wouldn't be any worse than it is now. Instead, we're sealed in some bizarre time paradox, where 1700's military thought governs 2010's social thought.
When Thomas Jefferson said things like "The dead shouldn't govern the living", and "there should be a revolution every 20 years", this is exactly what he was afraid of: An increasingly obsolete set of laws inflexibly enforced in a completely different world. I'm not saying we should reinterpret the Second Amendment out of existence, I'm saying we should make laws to suit the world we live in through amending the Second Amendment.
It's what The Founders wanted, after all.
*The fact that the word "Welfare" is in the constitution is why FDR and his policies used the word. Republican's ability to turn the purpose of our government into a dirty word shows that there is no such thing as a liberal media.
** I've always found "The Founders" to be an irritating phrase. It's fine to refer to "The Founders" as a group of people that fought over the creation of the country, but that's it. It is never appropriate to say that "The Founders believed X". Any room that has Alexander Hamilton and Thomas Jefferson would have trouble reaching consensus on a resolution on anything. Anyone who wears the cloak of "The Founders' Intent" to mask their actions is a cad.