Although I believe people do have a right to provide for their own defense -- and thus have a right to keep and carry some sorts of weapons -- I'm not opposed to all weapons restrictions. For instance, an argument that may support my right to carry a concealed handgun may not have much weight when I start building a nuclear device next-door. But why not? If my neighbor has a nuclear device of his own, I'll need one too to deter him. Right?
Well, I don't think so, and here's why. As fine a job as many police forces do, their primary purpose isn't to prevent all crime, but to increase the cost of committing crimes. In contrast, an individual's (rightful) primary purpose (if they so desire) is to completely prevent all crimes against their person and their family. The police simply cannot be everywhere all the time, ready to prevent every conceivable crime, and it's not even their job to do so. Most police are under no legal obligation to intervene if they see a crime in progress (link anyone?). Their job is to reduce aggregate crime; although they do that by catching specific criminals, police very rarely actually stop a crime in progress.
Extrapolate those thoughts about the police to the government as a whole, and I think you'll see where I'm going. We as individuals grant some of our right to use force to the government and entrust it with the authority to reduce crime (and even fight wars) on the large scale. It's more efficient and more effective to field an Army division than to field 20,000 individuals. When it comes to large scale violence, the government always knows where, when, what, who, &c., and is generally able to respond within a useful timeframe.
Howver, when it comes to small-scale violence, the police are generally nowhere to be found till after the fact, and it's impossible to envision any alternative system. For that reason, individuals must retain the power and authority to protect themselves from small-scale violence. I know what's happening to me, because I'm there.
Of course, crime-prevention is only half the story behind the 2d Amendment -- the right to keep and bear arms is also important because the populace should have the power to protect itself from a tyrannical government, by force if necessary. So should I have the right to own a nuclear weapon to deter the feds? Well, considering the massive infrastructure that would be required to maintain a useful weapon and delivery system, this option is probably entirely impractical (except perhaps for Bill Gates).
A decent argument might be made for arming state troops with nuclear weapons, but is there really a point? When it comes down to it, the American military is made up of common citizens from all parts of the country, not tribe-or ethnic-based conscript units as are found elsewhere in the world. Our best protection from a military coup or a tyrannical government is the simple fact that our soldiers and officers wouldn't obey an order to nuke their own city. Unlike in many parts of the world, the government is made of us, and there is no real them. Sure, there are ideological differences, but none of them are fixed across time, and most families have members from just about every side of the spectrum.
So-called "liberals" who want to restrict individuals from owning small-arms are living in a fantasyland in which an omnipresent, omnipotent, benevolent government solves all our problems. On the other hand, some libertarians I know who advocate completely unrestricted weapon rights are missing the point of liberty also: are you less free now than you'd be if all your neighbors had nuclear weapons?