Jeffrey Toobin makes a great point:
Conservatives often embrace “originalism,” the idea that the meaning of the Constitution was fixed when it was ratified, in 1787. They mock the so-called liberal idea of a “living” constitution, whose meaning changes with the values of the country at large. But there is no better example of the living Constitution than the conservative re-casting of the Second Amendment in the last few decades of the twentieth century...
The re-interpretation of the Second Amendment was an elaborate and brilliantly executed political operation, inside and outside of government. Ronald Reagan’s election in 1980 brought a gun-rights enthusiast to the White House. At the same time, Orrin Hatch, the Utah Republican, became chairman of an important subcommittee of the Senate Judiciary Committee, and he commissioned a report that claimed to find “clear—and long lost—proof that the second amendment to our Constitution was intended as an individual right of the American citizen to keep and carry arms in a peaceful manner, for protection of himself, his family, and his freedoms.” The N.R.A. began commissioning academic studies aimed at proving the same conclusion. An outré constitutional theory, rejected even by the establishment of the Republican Party, evolved, through brute political force, into the conservative conventional wisdom.
And so, eventually, this theory became the law of the land.
What was the previous prevailing theory? That the Second Amendment is about what it says it's about in the first sentence: the right of individual states—not private individuals—to have a "well-regulated militia."