Make sure you’re sitting down.
To summarize, we conclude that the Second Amendment protects an individual right to keep and bear arms. That right existed prior to the formation of the new government under the Constitution and was premised on the private use of arms for activities such as hunting and self-defense, the latter being understood as resistance to either private lawlessness or the depredations of a tyrannical government (or a threat from abroad). In addition, the right to keep and bear arms had the important and salutary civic purpose of helping to preserve the citizen militia. The civic purpose was also a political expedient for the Federalists in the First Congress as it served, in part, to placate their Antifederalist opponents. The individual right facilitated militia service by ensuring that citizens would not be barred from keeping the arms they would need when called forth for militia duty. Despite the importance of the Second Amendment’s civic purpose, however, the activities it protects are not limited to militia service, nor is an individual’s enjoyment of the right contingent upon his or her continued or intermittent enrollment in the militia.
And here’s an argument made better than I’ve heard gun rights advocates make it:
When we look at the Bill of Rights as a whole, the setting of the Second Amendment reinforces its individual nature. The Bill of Rights was almost entirely a declaration of individual rights, and the Second Amendment’s inclusion therein strongly indicates that it, too, was intended to protect personal liberty. The collective right advocates ask us to imagine that the First Congress situated a sui generis states’ right among a catalogue of cherished individual liberties without comment. We believe the canon of construction known as noscitur a sociis applies here. Just as we would read an ambiguous statutory term in light of its context, we should read any supposed ambiguities in the Second Amendment in light of its context. Every other provision of the Bill of Rights, excepting the Tenth, which speaks explicitly about the allocation of governmental power, protects rights enjoyed by citizens in their individual capacity. The Second Amendment would be an inexplicable aberration if it were not read to protect individual rights as well.
The whole decision, in PDF format, here.