Kevin Williamson (with whom I disagree vehemently on illegal immigration) explains, correctly:
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. Consider the words of Supreme Court justice Joseph Story — who was, it bears noting, appointed to the Court by the guy who wrote the Constitution:
The importance of this article will scarcely be doubted by any persons, who have duly reflected upon the subject. 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.
“Usurpation and arbitrary power of the rulers” — not Bambi, not burglars.