The purpose of having citizens armed with paramilitary weapons is to allow them to engage in paramilitary actions. The Second Amendment is not about Bambi and burglars — whatever a well-regulated militia is, it is not a hunting party or a sport-clays club. It is remarkable to me that any educated person — let alone a Harvard Law graduate — believes that the second item on the Bill of Rights is a constitutional guarantee of enjoying a recreational activity. There is no legitimate exception to the Second Amendment for military-style weapons, because military-style weapons are precisely what the Second Amendment guarantees our right to keep and bear. The purpose of the Second Amendment is to secure our ability to oppose enemies foreign and domestic, a guarantee against disorder and tyranny.
He goes on to cite Joseph Story, a Supreme Court justice from 1811 to 1845:
The militia is the natural defence of a free country against sudden foreign invasions, domestic insurrections, and domestic usurpations of power by rulers. It is against sound policy for a free people to keep up large military establishments and standing armies in time of peace, both from the enormous expenses, with which they are attended, and the facile means, which they afford to ambitious and unprincipled rulers, to subvert the government, or trample upon the rights of the people. The right of the citizens to keep and bear arms has justly been considered, as the palladium of the liberties of a republic; since it offers a strong moral check against the usurpation and arbitrary power of rulers; and will generally, even if these are successful in the first instance, enable the people to resist and triumph over them.
Obviously, Joseph Story was wrong. Militia are hopelessly inadequate as a means of defending a free country. While "people's war" militia-based strategies have been employed to wear down invading armies in numerous countries over the past century, not one of those countries (Vietnam, Afghanistan, Somalia, Iraq, southern Lebanon, etc) is "free". This is not an accident of history. Freedom is the product of orderly democratic governance and the rule of law. Popular militias are overwhelming likely to foster not democracy or the rule of law, but warlordism, tribalism and civil war. In Lebanon, Pakistan, the Democratic Republic of the Congo, Mali, Colombia, the Palestinian Territories and elsewhere, we see that militias of armed private citizens rip apart weak democratic states in order to prey upon local populations in authoritarian sub-states or fiefdoms. Free states are defended by standing armies, not militias, because free states enjoy the consent of the governed, which allows them to maintain effective standing armies. Like every other free country apart from Costa Rica, the United States has a standing army in times of peace, and has since 1791, when the founding fathers realised a standing army would be necessary to fight the irregular popular militias of the continent's Native American peoples. (Guess who won?)