Here’s a little mental experiment. Imagine, for a moment, that the Tsarnaev brothers, instead of packing a couple of pressure cookers loaded with nails and explosives into their backpacks a week ago Monday, had stuffed inside their coats two assault rifles—Bushmaster AR-15s, say, of the type that Adam Lanza used in Newtown. What would have been different?

Well, for one thing, the brothers would probably have killed a lot more than three people at the marathon. AR-15s can fire up to forty-five rounds a minute, and at close range they can tear apart a human body. If the Tsarnaevs had started firing near the finish line, they might easily have killed dozens of spectators and runners before fleeing or being shot by the police.

The second thing that would have been different is the initial public reaction. Most Americans associate bomb attacks with terrorists. When they hear of mass shootings, they tend to think of sociopaths and unbalanced post-adolescents. If the Tsarnaevs had managed to carry out a gun massacre unharmed and escaped, their identities unknown, would the first presumption have been that the shooters were Islamic extremists? Or would people have looked in another direction?

Third, had the attack been carried out with assault rifles rather than explosives and nails, the gun-control bills that perished on Capitol Hill just two days after the Boston bombings may have met a different fate. After yet another gun massacre, this one on the streets of Boston, it’s hard to imagine the White House wouldn’t have been able to summon up sixty votes in the Senate for expanded background checks. The proposed ban on assault weapons would surely have gotten the support of more than forty senators, too, and the proposal to ban multi-round magazines would also have gained more support—that’s if the gun lobby hadn’t managed to postpone the votes until emotions had cooled, which it would certainly have tried to do.

Finally, there’s the question of what would have happened to the Tsarnaevs after they had been caught—that’s assuming one or both of them had survived the attack. Just for the sake of argument, let’s say things had developed pretty much as they did, with Tamerlan, the elder brother, being killed, and Dzhokhar, the younger brother, being wounded and captured. Would the government have charged him with conspiring to use “weapons of mass destruction,” a count that could lead to the death penalty? And if they had done this, what would it have meant for the future of assault weapons? Once they’d been classified as W.M.D.s, would that have not made a difference to the public debate about how freely available they should be?

Yes, this is only a counterfactual exercise, which, like all such riffs, shouldn’t be taken too literally. But it’s hard to think about it for long without coming to the conclusion that there’s something askew with the way we think about and react to various types of extreme violence, and the weapons used in such episodes. In a country where each life (and death) is supposed to count equally, surely the victims of gun violence should be accorded the same weight as the victims of bomb violence. And the perpetrators should get equal treatment, too. But, of course, that’s not how things work.

Let me make clear that I am not trying to equate, in any moral or legal sense, mass shootings that result from personal vendettas or psychological pathologies with acts of terrorism carried out for political purposes. Nor am I suggesting that the Tsarnaevs can’t be classed as terrorists. From what has appeared in the media, it appears that Tamerlan had been frequenting radical Islamist Web sites, which promote conspiracy theories about 9/11 and violence against the West. (The motivations of Dzhokhar, whose main passion at UMass appears to have been pot, remain murkier.)

My point is about perceptions and reality, and how the former can shape the latter. The Tsarnaevs did have at least one gun—evidently a pistol, rather than the mini-arsenal originally reported—which they apparently used to kill an M.I.T. police officer, but that wasn’t what kept an entire city locked indoors: it was the fact that there were “terrorists,” who had carried out a bombing, on the loose. As I pointed out the other day, numerically speaking, terrorism, especially homegrown terrorism, is a minor threat to public safety and public health. It pales in comparison to gun violence.

Set off in a public space a couple of crude, homemade bombs that you appear to have made using a recipe on the Web, and the state will make you Public Enemy Number One. To insure that you are caught and punished, there are virtually no lengths to which the authorities won’t go. They’ll assemble a multi-agency task force overnight, calling on some of the enormous investments in hardware, intelligence, and manpower that have been made since 9/11. They’ll haul in anybody who might be remotely connected to the crime scene, and, if necessary, shut down an entire city. Once you’re caught, they’ll interview you in your hospital bed without reading you your legal rights and then charge you with using W.M.D.s. If you weren’t born in this country, there will even be talk about changing the immigration laws.

If you systematically shoot a classroom full of defenseless six-year-olds and blow off your own head, things proceed rather differently. To be sure, you, or your memory, will be hated and vilified. But the political system, in hock to the N.R.A., will classify you as a nut whose deadly actions have few or no policy implications. (With the demise of the gun-control legislation, that’s what it did with Adam Lanza.) Life and politics will go on as normal. The President will probably visit the scene of your outrage and say consoling things to the families of your victims. He’ll mean what he says, but he won’t be able to do much about it, and nobody will ask why the F.B.I. or the C.I.A. didn’t realize you were such a menace to society and lock you up preëmptively. Crazed shooters, after all, are something we’ve grown used to.

Because we have become inured to deaths from shootings, and because of the association of guns and liberty in the minds of many Americans—an association assiduously promoted by the gun lobby—the political system no longer responds to gun deaths. Terrorist acts, on the other hand, even ones masterminded by Mutt and Jeff from Cambridge rather than Osama and K.S.M. from Tora Bora, still have the power to spook the nation and swing the entire U.S. government into action.

Which is what got me thinking in the first place about what would have happened if the Tsarnaevs had been shooters rather than bombers. Maybe I am wrong about how things would have played out. But the country, or large parts of it, would finally have been forced to confront its cognitive dissonance about gun violence and terrorism, which, at the very least, would have been educational.

As it is now, the law-enforcement agencies are patting themselves on the back; Tsarnaev is headed for court; and in Washington the policy debates look set to continue along their well-established and ossified tracks. Meanwhile, the rest of the world looks on in astonishment at a country that so vigorously confronts one source of death and destruction while turning its back on another.

Note: In the original post, I wrote that the semi-automatic AR-15 was capable of firing forty-five rounds a second, which is obviously wrong. I meant to say that it was capable of firing forty-five rounds a minute. Sorry for the error, which I’ve corrected.

Photograph by Kevork Djansezian/Getty