The Second Amendment to the U.S. Constitution reads:
"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."
George Mason's original draft reads:
"That the People have a Right to keep and to bear Arms; that a well regulated Militia, composed of the Body of the People, trained to arms, is the proper, natural, and safe Defence of a free State; that Standing Armies in Time of Peace are dangerous to Liberty, and therefore ought to be avoided as far as the Circumstances and Protection of the Community will admit; and that in all Cases, the military should be under strict Subordination to, and governed by the Civil Power."
Mason's Virginia Declaration of Rights had put it this way 12 years earlier:
"That a well-regulated militia, composed of the body of the people, trained to arms, is the proper, natural, and safe defense of a free state; that standing armies, in time of peace, should be avoided as dangerous to liberty; and that in all cases the military should be under strict subordination to, and governed by, the civil power."
The Right-To-Bring-Assault-Weapons-to-School Second Amendment turns out to have its origins in an attempt to avoid maintaining standing armies. In place of standing armies, the states of the new United States were to create well-regulated militias. The first half of the Second Amendment explains why people should have a right to bear arms:
"A well regulated Militia, being necessary to the security of a free State ... "
Bearing arms in a well-regulated militia did not mean bearing guns that can reliably shoot well, since such didn't exist. It certainly didn't mean bearing guns that can kill entire crowds of people without reloading. It didn't mean bearing arms outside of the well regulated militia. Much less did it mean bearing arms in school and church and Wal-Mart.
By "free state" many supporters of this bill of rights meant, of course, slave state. And by "people" they meant, of course, white male people -- specifically people who would be taking part in well regulated militias.
The Third Amendment reads:
"No Soldier shall, in time of peace be quartered in any house, without the consent of the Owner, nor in time of war, but in a manner to be prescribed by law."
The Second and Third amendments originated as restrictions on what we would later create and come to call a Military Industrial Complex, a permanent war machine, a federal tool of abusive power.
The militias of the Second Amendment are meant to protect against federal coercion, popular rebellions, slave revolts, and -- no doubt -- lunatics who try to mass-murder children.
The descendants of those militias that we call the National Guard are meant, in contrast, to recruit ill-informed young people who imagine they'll be rescuing hurricane victims into endless occupations of oil-rich lands far from our shores.
To comply with the Second Amendment we must end federal control over the National Guard, regulate such state militias and police forces well, regulate their weapons well, and deny such weapons to all others and for any other use.