It is popular for certain politicians and others of dubious qualification to ask "Why does anyone need an assault rifle?" In response, let's first get down to fundamentals.
The USA prides itself on being a "free" country. And individual liberty IS our heritage and is the dominant theme in the writings of the Founders and in the founding documents. It is clear that our ancestors had in mind that Americans would enjoy the freedom that has been so often denied to the people of Europe and elsewhere. Indeed, for most of the Founders, protecting individual freedom was the primary purpose of government. Freedom is our political heritage and a core value in our cultural identity. As an American, it is not necessary to justify freedom. Freedom is a laudable end in itself. Freedom is its own justification.
It is those who would curtail freedom that must justify their actions. "Why do you need your freedom?" should be an absurd question to any American, to any human being that hasn't had their spirit crushed by oppression. And so the question "why do you need an assault rifle?" is never an appropriate threshold question for this discussion. Although the cause of liberty has suffered much since the US Constitution was ratified, we have not reached the point where the default position for policy discussions is denial of freedom, with the enjoyment of freedom to be "granted" by the government only on an "as needed" basis.
So, indeed, "why do you need an assault rifle?" is an absurd and subtly tyrannical question. The proper threshold question goes the other way, from the free man to his government: "what is your compelling reason for denying my freedom?" So let's start by addressing this PROPER question. Since we presumably want to address the question rationally, let's skip over the emotional rhetoric and hysterical pronouncements of the mass media and politicians and go right to the actual facts.
According to FBI statistics, violent crime, including murder, has been FALLING. In fact, it has been falling steadily for over twenty years. It has fallen by more than fifty percent since it peaked in the early 90s. That is an astonishing reduction. Hmmmmm . . . not really a compelling case for infringing on the freedom of innocent people, is it?
But certainly those evil-looking weapons of war are a menace? Well, no. In 2010, for example, there were 12,996 murders in the USA (again citing FBI stats). Of those murders, firearms were used in 8,775. Of those, a whopping 358 of those murders were committed with rifles. And only some unknown fraction of those rifles were of the "evil" military style. And even a smaller fraction of those were aided by large capacity magazines. The plain fact is that military style rifles with high capacity magazines make a negligible contribution to violent crime even though literally millions are in the hands of American civilians. The ONLY reason such rifles are singled out for abuse is because they look menacing. Clearly, if you want to be rational about it, even if you insist on blaming hardware for violent crime, you need to look somewhere other than rifles.
So, having utterly failed to provide a compelling reason to deny people their freedom to own a rifle, the argument should be over. However, since the average media dupe is going to insist that "something must be done" in response to the tragedy of Sandy Hook, even if it is irrational, let's inquire further.
The Sandy Hook shooter murdered his mother, stole her lawfully owned gun, and killed a bunch of children with it. Exactly what kind of gun law would have prevented this tragedy? Since he didn't purchase the gun, but stole it, no controls on purchasing, no licensing requirement, no background check, would have stopped him. Even a complete ban on future sales of such weapons would not have stopped the killing since he stole an existing weapon rather than buying a new one. So EVERY law being proposed in the wake of the shooting would have failed to prevent the shooting. This is typical for legislation based entirely on emotion.
But suppose you eliminated ALL civilian ownership of guns through a massive confiscation? You still would have the possibility of acquisition by theft and diversion from approved holders (such as the police) and, more importantly, through the black market and smuggling. Mexico and other similar countries are awash with firearms in spite of a near total ban on civilian ownership (and not just the guns provided to the drug gangs by the U.S. government).
In the US, a total gun ban can reasonably be anticipated to be no more successful than has been the ban on marijuana, which manages to slip through by the bale on a daily basis in spite of the fact that it can be detected by trained dogs (guns cannot). Additionally, a bale of marijuana goes up in smoke in a matter of weeks and must be replaced. A gun, once smuggled into the country, lasts essentially forever. While it might be theoretically possible to convince the law-abiding to disarm themselves, only a fool thinks criminals will comply or be unable to acquire all the weapons they want through the black market. Government cannot even keep weapons out of U.S. prisons. How much less likely is it that they could control the flow in the country at large?
But suppose (against all the evidence) you COULD keep guns entirely out of civilian hands. What would stop a homicidal/suicidal maniac from killing people with explosives like the Bath Michigan school bombing that killed 38 children (http://en.wikipedia.org/wiki/Bath_School_disaster), or with gasoline like the Happy Land arson fire that killed 87 people (http://en.wikipedia.org/wiki/Happy_Land_fire) or with a knife like the recent attack on children in China (http://www.thenewamerican.com/usnews/crime/item/13946-china-school-knife-massacre-why-no-media-coverage). Unarmed human beings are relatively fragile and it is not a difficult task to find ways to harm them, especially if you intend to die yourself in the process. It is simply folly to think you can make the world safe from homicidal maniacs by passing a law.
Indeed, no gun control law has ever been demonstrated to reduce violent crime. In fact, in the USA, the stricter the gun control, the greater the levels of violent crime. It is virtually a perfect correlation. As rational observers, we cannot assume that correlation equals causation and so we must admit the possibility that those jurisdictions have the most gun control BECAUSE they have the most violent crime and not the other way around. But we CAN conclude that gun control has not been effective at curtailing violent crime where it has been enacted. Not in the USA and not in the much vaunted UK, where gun control has not been followed by a reduction in violent crime, but rather by an increase. Australia and New Zealand have had the same result.
Gun control as a tool of crime control does not survive rational scrutiny and has a dismal track record in the real world. But since we have come this far, let's go all the way. Why not go ahead with a futile program? It certainly would not be the first. Indeed, government in the USA consists almost entirely of futile, expensive programs that curtail freedom. So why not another one? Why such resolute resistance to another useless government program that chips away at freedom?
The Founders of this country knew their history. They knew tyranny. They knew that ANY government could become tyrannical. And they desperately wanted the government they were creating to NOT become tyrannical. The Constitution goes to great lengths to try and erect barriers to tyranny by limiting the government to enumerated powers, by spreading its powers around among the people, the states, and various branches of the government, all with powers to check the others. And then on top of that they applied a list of restrictions, in the form of the Bill of Rights, that were based on their experience with the typical operations of tyrants. The Founders knew that government was the ultimate enemy of liberty, that it was, as George Washington said, "Like fire,  a dangerous servant and a fearful master."
And so, when we get right down to the nitty gritty, the somewhat embarrassing answer to the question "why do you need an assault rifle?" is: "we don't trust our government not to try to enslave us and armed resistance is the only guarantee against it." And there is plenty of history to back up the mistrust.
In the last century, governments around the world have slaughtered at least 100 million of their own people. This has happened under the auspices of many cultures, many races, and many forms of government. Anyone who responds to this with the idea that we are somehow immune to murderous tyranny should now be hearing the scoffing laughter of Santayana in their ears. You ignore history at your peril. Gun owners are not going to ignore history. "Oh, but we have democracy!" Yes, and Hitler was elected. Stalin and Mao probably would have been if they had bothered with elections, but they didn't bother because they had all of the guns.
The more sophisticated advocate of gun control will not make the foolish assertion that "it can't happen here" but will instead assert that resistance is futile because there is nothing the people can do against the might of government should it decide to enslave them. I'm always a little at a loss at this argument as the implication seems to be that since we are no match for the military, we should FURTHER disarm ourselves and not even try to defend freedom should it come under attack. I wish such people happiness in their chains.
Aside from being pathetic and cowardly in their willingness to abandon their freedom, and the freedom of their children, without even raising a fist, they are also wrong on the facts. The Soviet Union, at the height of its powers, was unable to dislodge the Mujahadeen from Afghanistan. Nor could the might of the US army dislodge the Viet Cong. And we calmed the insurgency in Iraq only through bribery (and only temporarily in my opinion). The historical fact is that a determined indigenous resistance, especially if well-armed, is ALWAYS a force to be reckoned with. And that is exactly why the Founders specifically denied the Federal government the power to disarm the people - because an armed population is a bulwark against tyranny. And that, sir, is why I need a rifle suitable for combat.
The gun control advocates may reject the idea that an armed population acts as a defense against tyranny or scoff that such is needed. That's fine. In a sense it doesn't matter whether they believe it or not. What matters, and it is critical that they understand this, is that millions of gun owners DO believe it. So when government aspires to take their guns, they see it as a direct attack on the most important of all restraints on tyranny. They see it as a direct attack on the most fundamental of ALL freedoms, the freedom that PROTECTS all the rest. They see the forces of tyranny trying to pull their “liberty teeth”, to quote General Washington again.
To the gun control advocate, gun control seems to merely be restricting a dangerous hobby. But to millions of gun owners, the very future of freedom, the future of their children, depends on them NOT allowing themselves to be disarmed. They see every attempt at licensing, registration, magazine bans, etc. as government eating away at the keystone of liberty. That civilian disarmament is the act of a tyrant has been demonstrated time and time again with catastrophic results. Whether Mr. Gun Control Advocate believes it or not does not matter in the end because gun owners do and they will not be convinced otherwise.
Most scholars agree that one of the reasons the American Civil War happened was because both sides underestimated the will of the other side to fight and die for the cause. Advocates of gun control think that passing a law banning guns or magazines will be a victory. They are wrong. It will, instead, be the beginning of a war in which good people will fight and die. The gun control advocate's refusal to understand that only makes the tragic outcome more likely. Let's not go there.