Libertarians disagree about many important things, including: whether even a minimal state is morally permissible; if so, what functions may it legitimately undertake; the morality of abortion; foreign policy; whether intellectual property rights are morally defensible, and so on. One thing we don’t disagree much about is the right of law-abiding citizens to possess firearms, and to carry and employ them for purposes of self-defense.
This position follows readily from the theory of natural rights. Before states were “invented,” persons were entitled to defend themselves against aggressors, acquire and dispose of property, enter into contracts, and so forth. Accordingly, it is clear that in a pre-political world, persons would be entitled to produce firearms and to sell them to willing purchasers, who could justly use them to defend their persons and property.
Because these rights predate states, the existence of political governance cannot call them into question. Indeed, states are morally legitimate (if at all) only to the extent they enhance the rule of law relative to what would exist in their absence, i.e. what philosophers call a “state of nature.” If a state fails in this obligation, citizens may invoke their natural rights and revolt, as we did against our colonial masters. Firearms would certainly prove useful in such a contingency.
In light of what has already been said, for libertarians the burden shifts to those opposed to gun rights to articulate compelling moral grounds for restricting their possession and use. The most obvious argument to this effect is that firearms are often the cause of deadly accidents, and are frequently used to commit crime, up to and including murder. But this a weak rationale on multiple grounds.
Since only a tiny fraction of murders are committed with so-called assault rifles or in mass killings, measures such as restricting the capacity of magazines used with semi-automatic weapons would have a negligible effect. To move the needle at all, something like the outright confiscation of most firearms would be necessary. Such an effort would be extremely expensive and of questionable efficacy, but I will not dwell on the problems of enforcement. Rather, I will approach this issue from a rights perspective.
Initially, note that those who believe in natural rights reject utilitarianism. Which is to say that we hold the view that rights should be respected even at some practical cost. For example, suppose that on a utilitarian calculation the costs of “hate speech” outweigh its benefits, we nevertheless endorse the right of persons to express distasteful thoughts. So too, innocent people have the right to possess and lawfully use firearms, even in the unlikely event (see below), that there is some net harm to society. I would change my mind if such costs were catastrophic, but they clearly are not.
According to FBI statistics, in 2014 (the most recent year available) 8,124 people were murdered by means of firearms in the US. Using some basic math, this means that that statistically a US citizen has a roughly .00003% annual chance of being killed this way. If you are not in a violent gang, your chances are much lower still, since you will not be shot dead by a rival criminal enterprise in a battle over “turf.”
Firearms also cause accidental deaths, on the order of about 700 year. But as Levitt and Dubner have shown in their bestselling Freakonomics (pp.149-50), a child is roughly 100 times at greater risk of accidental death from visiting a friend’s house that has a residential pool, than one that contains a firearm. And, there is no political movement for “pool control.”
Of course, I have so far omitted the benefits of firearm ownership, which are numerous and substantial, but difficult to quantify. Obviously, people desire to own firearms, as about 40% of US households possess at least one. The reasons are many.
First, guns are used to stop crimes in progress, and to deter them. It is a contentious subject, but a panel appointed by President Clinton estimated in 1994 that guns were used to thwart crimes some 1.5 million times annually. Furthermore, very large numbers of Americans have decided that they don’t wish to rely solely on the police if they are awakened at 3 am by someone breaking into their residence, and this feeling of enhanced security clearly has utilitarian value. On top of this, many millions of Americans enjoy hunting, target practice, and participating in competitive shooting sports.
One of the obvious weaknesses of utilitarianism as an ethical theory is that, given the limits of social science research, it is generally impossible to settle policy questions of this sort empirically. There are simply too many variables to be controlled, and they must be assigned weights which themselves cannot be justified. Libertarians of all stripes believe that, at a minimum, there is a heavy presumption against state action that limits the liberty of citizens. And it is quite clear that this burden cannot be sustained in the case of firearm ownership.
 There are no doubt some firearms regulations that are consistent with libertarian principles, i.e. those that would measurably enhance public safety while not unduly burdening the rights of gun owners. For example, the private, unlicensed ownership of fully automatic, belt-fed machine guns, does little or nothing to enhance a person’s legitimate right of self-defense, yet might put innocent people at undue risk of attack by terrorists or the mentally ill.
 Gun control advocates mock this argument by observing that a citizenry armed with small arms would have no chance against a modern military, with its tanks, fighter jets, etc. But guerrillas don’t fight pitched battles, and many successful insurgencies have started out with small arms or even less.
 According to FBI statistics, there were 248 murders committed with all types of rifles in 2014 (the most recently reported year).
 The majority of gun-related deaths are suicides, accounting for roughly 21,000 fatalities in 2014. Without in any way discounting the tragedy of such deaths, persons have this right, and so I do not count this as a societal cost. It may be the case that the availability of firearms enables some mentally incompetent people to commit suicide who might otherwise be prevented from acting. However, there are many ways to die, and thus I believe it pure speculation to assert that even an outright ban on private ownership of firearms would save many lives in this fashion, or affect my argument.