The Second Amendment to the U.S. Constitution states the following: "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." But what that means is the subject of intense debate. Pro-gun partisans argue that the Constitution's framers guaranteed peoples' right to possess and carry just about any sort of firearm. Gun control advocates say it was intended to allow states to maintain the equivalent of today's National Guard units [source: Krouse].
But as Supreme Court Justice Charles Evans Hughes once noted, "The Constitution is what the judges say it is" [source: Columbia University]. And so far, probably to both sides' frustration, the courts have never fully defined the Second Amendment and its implications. Instead, the U.S. Supreme Court has issued a series of rulings that mostly have upheld the government's authority to impose restrictions upon weapons.
For example, in the 1937 case U.S. v. Miller, a court upheld a federal statute requiring licensing of sawed-off shotguns, saying that some sorts of weaponry weren't needed by a militia and thus weren't constitutionally protected. (Gun rights advocates replied that this type of weapon had been used by militia before.) More recently, in the 2008 case District of Columbia v. Heller, the court found that citizens did have a right to possess handguns at home for self-defense. But the justices said the government still could impose other limits -- such as banning criminals and mentally ill people from owning guns, regulating gun sales and barring guns from schools and other places [source: Krouse].