Self-Defense and Guns
In discussing the issue of retaliatory force, we can see why a government is necessary to clarify when a use of force is appropriate or not. This is certainly useful before the force has been used, so that you can avoid any inappropriate use and potentially being retaliated against.
The government may do fine with using retaliatory force after a crime has been committed, but that's not very comforting for someone who's life is in danger during an actual crime. The police can't be there to defend you, so you have to be able to do it yourself. While most acts of retaliatory force have to be okayed or go through the government itself, self-defense must be left to the individual.
Self-defense has a number of requirements. First, the individual must have the means to protect himself. In today's world, firearms are the best means of accomplishing that protection. A gun is a great equalizer. Without a gun, physical strength may decide who wins or loses. A gun makes physical strength a non-issue.
Consequently, the right to gun ownership is important. Governments typically oppose this because a well-armed populace is harder to control. There are also unfounded myths like gun control reduce violent crimes. There are many books on gun control vs. gun ownership, although Objectivists generally side with gun ownership. The Objectivist position stems from the right to self-defense, which means allowing people to own the tools that will really protect them.
Self-defense also requires the government have clear methods of determining when you're able to use force in self-defense, and how much force is appropriate. This usually involves the threat of physical harm, or the reasonable belief that there is such a threat. Under correct conditions, a person is able to defend himself, others, and potentially his property.
Self-defense has some potentially interesting legal aspects. The government usually has evidence that you've killed or injured someone, and you have to prove that it was in self-defense. Where does the burden of proof rest? Normally, the burden of proof is on the government to show beyond a reasonable doubt that you've committed a crime. But when you make the assertion that it was self-defense, the burden of proof falls to you. What standards does a government use to determine guilt in this kind of case? That's beyond the scope of this discussion, but you can see that self-defense can be treated in a non-standard way in the legal system.
I can't discuss guns without bring up another important implication of widespread gun ownership. One method of ensuring that a government does not become tyrannical is widespread gun ownership. The government would have to try to fight and control an armed population, which is virtually impossible. Yes they could win any direct skirmishes, but controlling a population that can shoot your administrators at any time is a significant task. The historic reality is that government disarm the population before becoming tyrannical as a practical necessity.
So widespread gun ownership not only allows individual self-defense, but it also has value in defending against government abuse. This is true for both foreign and domestic governments. An invading army would have a difficult time occupying an armed nation that wants to resist.