Yes, this post is going to be about guns.
Let me start by stating three obvious things:
First, if the gun had never been invented there would be no mass shootings using guns. Similarly, if guns were effectively removed from a society completely, the same would result.
Second, guns are not the only weapon which have been used in mass attacks - other weapons have ranged from fertilizer-based "IED"s to planes.
Third, if a civilian - say, one of the teachers who died in this most recent shooting - had been armed, trained, and firing back, the casualties would have been far lower: resistance would at least have slowed to gunman.
The Facts (As Best as I Can Tell)
It is sometimes supposed that, since modern weapons are so much more effective than 18th century weapons, different principles must apply. Here is an example of the question, posed in large friendly letters (but unfortunately without saying, "Don't Panic"). In fact that comparison gives about the highest possible potential for a musket, which - due to size and muzzle reload - would not actually have been at all a practical short-range weapon. It would be more accurate to compare the potential of a semi- or full-automatic weapon with the damage of a saber or bayonet, which would make the contrast even more stark.
In countries which have followed this logic, it is certainly the case that mass attacks with firearms are much rarer. Not unknown, as events in Norway last year showed, but relatively rare. And attacks made with other weapons do less damage - as argued by a CNN piece here.
On the other hand, the numbers of guns in a country does not demonstrate any particularly strong correlation to overall murder rates. If we can trust wikipedia more or less here, we find that the highest homicide rates belong to countries with underdeveloped economies and infrastructure. Although the page on number of guns is probably less accurate, these countries with high murder rates are fairly low on the list of gun possession - which seems to be more strictly a count of all guns, but this does not really affect the point. This lack of correlation is summarized by the Guardian here.
That point is this: while modern weapons make the potential for catastrophic crime much higher, they have little effect on people's willingness or ability to kill each other.
There are two reasonable solutions proposed. The first is that more people should probably go armed; the second is that as much as possible no one should be armed. Either one, if carried out extensively, would probably have some good effects. The first principle, deterrence, seems intellectually unpopular today, but its common sense is, I think, obvious: the armed woman is in less danger - being herself more dangerous - than the unarmed man. The second, although it has seen success in countries with strong centralized governments and populations willing to accept regulations, is not really in line with traditional American thinking on weapons. Further, gun control may not be the only issue in play: various outlets have recently highlighted problems with American attitudes and methods for dealing with mental issues present in many shooters. All of this is seen argued, for example, here.
The American Legal Situation
The American nation, founded as it was as a direct result of revolt against previous government, was naturally more concerned with maintaining independence and its citizens' rights than with potential problems that might arise with higher technology - and also belonged thoroughly to the school of deterrence. It is worth noting two things about the American Revolution's armies. First, the revolutionists immediately went and found all the cannon they could, in order to face the British troops on equal footing. Second, the guerrilla tactics used in the southern and western arenas were possible due to better marksmanship - and sometimes, with early rifles, better weapons - than the official army.
This experience and concern naturally led to the inclusion, very quickly, of the Second Amendment to the Constitution. True, its reasoning is somewhat oddly phrased, not to say ambiguous (does the "Security of a free State" refer to foreign aggression, tyrannical government, or both?); the early modern reliance on militia seems strange to most of us in Western culture - but it should be noted that this is after 150 years of one war or another has made the part-time soldier seem obsolete. Not exactly a ringing commendation of our cultures. But its requirement that, "the right of the people to keep and bear Arms, shall not be infringed," is clear. Together with the Fourteenth Amendment, which places similar restrictions on individual state governments as the Constitution and its first amendments placed on the Federal government, there is virtually no way an American governmental body above the county level can legally restrict any weapons.
This is to say, unless the law is changed - in this case, an amendment would be legally required, though I belong to that class of people who suspect this requirement will be ignored - the recourse in the United States has to be to deterrence; disarmament is not a feasible option when others can freely find weapons.
The question remains whether gun control is in fact an effective solution. I find myself sympathetic to the argument; and in fact I have never felt any need to carry weapons myself. On the other hand, it has the difficulty of the universal negative - it is hard to "prove", that is, enforce - and it still leaves the question of the citizens' relationship to government. I will honestly admit that I see governments, in the US and across the world, growing and regulating and intruding, and I believe eventually this will provoke revolt. In that case, I would like to have the recourse to defend myself - from one side or the other or both. And in the meantime -
Well, Japan and Switzerland have comparably low rates of violent crimes. What I believe this really points at is the necessity of a cohesive society; the US is currently looking at unintegrated "multiculturalism". (Yes, I am suggesting that the crime rate would probably fall if English was the official language of schools and the Constitution was taught as the next best thing to a secular gospel.) I also suspect the necessity of strong local - that is, available - government. The modern trend is towards top-heavy micromanagement, when what I would really like is for city hall to be on top of things.