On Tuesday, MSNBC’s Joe Scarborough offered his opinion on why some Americans buy semi-automatic rifles, and why they shouldn’t be allowed to have them:

For those that would resist any attempts to limit the madness of the proliferation of guns across this country, that one point they always seem to have on their side is well, all the things you're asking for, Joe, wouldn't have stopped Newtown; all the things you're asking for, Joe, wouldn't have stopped Las Vegas; all the things you're asking – which, I think in many cases, they may have very good points. But you talk to any law enforcement officer, you talk to Bill Bratton, you talk to people that we praise every day for keeping us safe, they all say there is no reason Americans should have military-style weapons in their possession – no reason whatsoever.

And after Newtown, I had several pretty tough debates with some very good friends. And at the end of the day, there's only one reason for somebody to have these weapons in their hands. Only one reason, and it's only one. They will not say it in a radio debate or a TV debate, but it's because they think the government's coming to get them, and they want to have a stockpile of weapons.

It's to kill American soldiers, or members of the American government.

Now, there are some people that just like collecting and shooting. Well fine, you can do what other countries do: register them, and put them in the gun clubs, where they stay there.

But why do we have a group of laws [where] a guy can have 20 weapons, 25 weapons like this, and literally kill – just go hunting for human beings like he did a couple of nights ago.

Although Scarborough puts it in crass terms, the Second Amendment was indeed conceived as a means of protecting citizenry from government oppression.

Professor Edward J. Erler of California State University, San Bernardino, writes that the original intent of the Second Amendment was to secure "first principles" and the "right of revolution."

And we must be clear – the Second Amendment is not about assault weapons, hunting, or sport shooting. It is about something more fundamental. It reaches to the heart of constitutional principles – it reaches to first principles. A favorite refrain of thoughtful political writers during America's founding era held that a frequent recurrence to first principles was an indispensable means of preserving free government – and so it is.

...Expressing a widely held view, Elbridge Gerry remarked in the debate over the first militia bill in 1789 that "whenever governments mean to invade the rights and liberties of the people, they always attempt to destroy the militia."

...The Declaration specifies that when government becomes destructive of the ends for which it is established...then "it is the Right of the People to alter or to abolish it, and to institute new Government." This is what has become known as the right of revolution, and a central ingredient of the social compact and a right which is always reserved to the people. The people can never cede or delegate this ultimate expression of sovereign power. Thus, in a very important sense, the right of revolution (or even its threat) is the right that guarantees every other right.

And if the people have this right as an indefeasible aspect of their sovereignty, then, by necessity, the people also have a right to the means to revolution. Only an armed people are a sovereign people, and only an armed people are a free people – the people are indeed a militia.

The declaration also contains an important prudential lesson with respect to the right to revolution: "Prudence...will dictate," it cautions, "that Governments long established should not be changed for light and transient causes." It is only after "a long train of abuses and usurpations pursuing invariably the same Object," and when that object "evinces a design to reduce [the people] to absolute despotism," that "it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security. Here the Declaration identifies the right of revolution, not only as a right of the people, but as a duty as well – indeed, it is the only duty mentioned in the Declaration.

The prudential lessons of the Declaration are no less important than its assertion of natural rights. The prospect of the dissolution of government is almost too horrible to contemplate, and must be approached with the utmost circumspection. As long as the courts are operating, free and fair elections are proceeding, and the ordinary processes of government hold out the prospect that whatever momentary inconveniences or dislocations the people experience can be corrected, then they do not represent a long train of abuses and usurpations and should be tolerated.

Many are unwilling to argue on this line, as Scarborough correctly noted, but it is indeed the primary reasoning behind the Second Amendment.