The gun debate is about power. It is important to recognize this central fact before we engage it, or we risk talking past one another. The introduction of modern weapon technology was a massive injection of power into our society. The entire gun control debate is about the wisdom of marginal adjustments to the allocation of that power.
Every act of weapons regulation is a reduction of the power of private citizens. This is its purpose. It is the best reason to support it; it is the best reason to resist it.
The American republic was designed with a decentralized and limited government, but it also exists, like all government, to hold power where private citizens holding absolute power produces evil. The best example of a necessary limitation of private citizens' power is in the police power of the state. Many of the oldest legal codes were developed to stop feuds and vigilantism by taking the power of criminal judgment and civil restitution out of the hands of individuals. The power to enforce the law rightfully resides exclusively with the state.
But the American system was designed to protect other powers from being allocated to the state. The bill of rights protects the power of free speech, assembly, press, and free exercise of religion. It also protects the power to keep and bear arms for a well-regulated militia. These powers are to be kept with the people to prevent the government from becoming self-serving. They ensure that when the rights of the people are limited, they are limited by the will of the people.
None of these powers, however, is absolute. One cannot evoke the First Amendment to defend his shouting "fire!" in a theater. Nor is a person's power to own a bazooka protected by the Second. While the powers to speak freely and to bear arms are generally maintained, they are and will always be subject to regulation. Some power must be taken from individuals for the health and safety of society.
So the question to ask with each proposed regulation is whether the power which the regulation would take from individuals is more like the power to shout "fire" in a crowded theater, or more like the power to express dissenting political ideas without fear of majoritarian oppression. This is an imprecise evaluation on which intelligent people of good will can disagree.
The question is not resolved by asking whether the power was known at the time the constitution was drafted. Blogging was not, but is rightly protected by the First Amendment. Nor is it simplified by asking whether the regulation is an encroachment on individual liberty. All regulation is, and much regulation is necessary. The issue can not be settled with dramatic rhetoric or appeals to the pain of those who have suffered from recent loss. It comes down to a rather mundane calculation about the good to be gained by marginal adjustments to the current allocation of power.
The power to own an automatic rifle allows people to kill effectively, efficiently, and quickly. Taking this power from free individuals is a serious thing. Not because this power serves some function -- the likelihood of a military dictatorship arising in the U.S. is about the same as the likelihood of a Zombie Apocalypse, and either way, an AR-15 isn't going to save you -- but because the loss of this power is an injury per se. The preservation of individual power is fundamental to every free society and a self-justifying good.
But that good might be outweighed by the evil that it facilitates. Human nature is complex and in places, dark. The preservation of individual power facilitates the imposition of an individual's will on the world. This is what allows great people to flourish and broken people to destroy.
Great people have flourished without bazookas and the power to yell "fire" in theaters. They will flourish without the right to keep an AR-15. So have broken people destroyed without bazookas and the power to yell "fire." So will they do without an assault rifle. But there is a chance that the destruction they wreak will be less terrible.
The loss of power in the individual that a regulation represents is an injury that everyone committed to the maintenance of a free society should be willing to acknowledge. I am convinced, however, that it is justified by the reduction in efficiency that it might cost those who would use it for evil.
Unlike the loss of individual power, the potential benefit to society from a new gun regulation is not a self-evident good. It must be convincingly demonstrated that the probable result of the regulation will outweigh the injury to individual power. Many people argue that it is futile to limit legal access to guns because criminals don't follow the law. I think that this hopelessness is a symptom of weak imagination. I have faith that our legislature can draft a law that will effectively make it more difficult to kill with military efficiency. And I think it is worth the cost of my power to do so for them to try.