The Second Amendment to the United States Constitution reads: "A well-regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
The Founding Fathers did not want a standing army. They believed it was too expensive. They had fought the American Revolution with militias made up of men with their own weapons, defending their own lands (that they stole from the natives who were on the land before them, but that's another issue for another day). They wanted people to keep their own weapons in case they had to call men to arms quickly to repel invasions. Communications weren't instant then. There were no radios, no aircraft, no motorized vehicles--no way to move troops quickly from a base to an invasion point. Having citizens (white male property owners over the age of twenty-one, at that time) own guns made sense, in terms of national security.
The times we live vastly different. We have a standing military. If we did have a military invasion, the average citizen would not be able to repel it; we would need the United States military arsenal. Perhaps there are people out there who think their personal arsenal of automatic weapons, if joined with other such arsenals, would be enough to repel an invasion, but this idea is pure folly. The most one could hope for out of several citizen-owned arsenals would be guerrilla warfare. The "well-regulated Militia" part is certainly outdated.
So...since the citizens no longer constitute a well-regulated militia, should we prohibit guns? Plenty of people do sport shooting on ranges, which does no harm to anyone. Some people hunt and eat the game animals they kill, which doesn't harm humans, and, in the case of deer, helps control a population of animals that do not have natural predators anymore (due to humans killing timber wolves, for example--yes, we're solving a problem we caused). Some people, such as real estate agents, who work on their own and don't know who they're meeting a good amount of the time, keep them for personal protection. Besides, the Constitution says the right of people to keep and bear arms shall not be infringed, right?
What the Constitution doesn't say is what type of arms the citizens may have. What's the limit? Is it wealth? The Koch brothers could certainly own their own private military, complete with aircraft--they have the money. Should they be able to hire mercenaries from Academi (formerly Blackwater, who killed citizens seemingly for sport in Iraq and confiscated weapons from citizens after Hurricane Katrina) and have a few fighter jets for air support? Should the average citizen be able to own a nuclear weapon? Chemical weapons? Biological? Should a wealthy person be able to hire a private navy, with ships that can hit targets several miles away? Should missiles be available to the average American?
Most reasonable people would put some limit on what arms should be available to the average citizen. Why then, should we not talk about regulations for gun ownership? Why not restrict the size of the magazines available for guns? Why not put mental health screenings in place as a requirement for gun ownership? Prohibition of guns could have the same result as the drug trade--in fact, those who have made billions from the drug trade could certainly diversify into illegal arms sales. However, nobody is talking about prohibition. Indeed, not many elected representatives are talking about imposing any limits--but perhaps it's time to talk about mental health screenings, in light of recent shootings in Aurora, Colorado and the Sikh Temple outside of Milwaukee, Wisconsin.
My personal opinion is that people with certain mental health issues should not have firearms, especially automatic weapons and other military-grade equipment. However, the Second Amendment, as many gun advocates will say, does not allow for an infringement on the right to keep and bear arms. Here's a compromise: let's do mental health screenings, but let people with mental health issues get guns. Then, put them on a watch list. Watch how many guns they buy, what kind of military-grade equipment they secure, and how much ammo they purchase. If they start accumulating the aforementioned items, put them under surveillance. When they leave the house with it, have a SWAT team or other law enforcement officials follow the person with guns pointed directly at his (because so far, all of these mass murderers have been male) head at point-blank range. If he continues to his destination, make sure the law enforcement officials take him down as soon as he pulls a weapon and points it at someone. He still owned his guns right up until he did something illegal, and now law enforcement call pull them from his cold, dead hands before he was able to do damage to a large number of innocent civilians.