The United States is peculiarly prone to mass killings by people armed with semi-automatic weapons. There is no good reason for ordinary citizens to own military-style weapons, and they have long posed a threat to American national security.

The mass murder of children at Sandy Hook Elementary School in Newtown, Conn., or the theater massacre in Aurora, Colo., both in 2012, were insufficient to spark a serious national legislative debate about this threat. Now that four Marines are dead at the hands of a civilian armed with such a weapon, can we discuss again a ban on those weapons, of the sort enacted in the Clinton administration?

Mohammod Youssuf Abdulazeez on Thursday rented a Mustang convertible and drove to a military recruitment facility in Chattanooga, Tenn., where eyewitnesses reported that he fired some 30 rounds from a rifle in just a matter of minutes. It sounded, one said, like a jackhammer. In other words, he used a semi-automatic weapon. He then drove seven miles to a reserve center for the Navy and Marines where he parked, propped up his assault weapon on the door of the car and opened fire, killing four Marines and wounding four others, including a policeman.

Also Thursday, a jury found James Egan Holmes guilty of first-degree murder for opening fire in a movie theater in Aurora, Colo. Holmes, a brilliant student, had long suffered from mental illness and was considered dangerous by his psychiatrist. On June 7, 2012, he bought a Smith & Wesson M&P-15 semi-automatic rifle from a Gander Mountain store in Thornton, Colo. He also bought a shotgun and two Glock semi-automatic handguns. He passed the background checks.

Over the Internet, Holmes purchased two magazine holders for the M&P15 and 3,000 rounds. That is, he modified the assault weapon so it could fire even more rounds without being reloaded. By the way, “M&P” stands for “military and police,” which was Smith & Wesson’s original intended clientele decades ago. In contrast, the M4 now used by many U.S. troops in war zones was originally envisaged as a lightweight weapon for police SWAT teams. With his modifications, Holmes may have been better armed than U.S. infantrymen in Afghanistan.

After Sandy Hook, the National Rifle Association (NRA), which is the publicity arm of a handful of big gun manufacturers who make billions selling military-style weapons to civilians, blamed video games. After the Charleston, S.C., black church shootings last month, the Confederate flag became the issue. Despite the Colorado jury’s rejection of Holmes’ insanity plea, his case was reported through the frame of his mental illness. Initial reporting on Abdulazeez in the Chattanooga shooting focused on his praise of jihad. One television commentator suggested bulletproof doors for military recruitment offices.

In other words, anything to avoid talking about the real issue, which is the startlingly easy availability of extremely deadly and powerful weapons.

In 1994, responsible national legislators including Sen. Dianne Feinstein of California crafted, and President Bill Clinton signed into law, a federal ban on the manufacture and sale to ordinary civilians of semi-automatic rifles and of ammunition drums with more than 10 rounds. Semi-automatic rifles fire every time the trigger is pulled (they are not machine guns, which can go on firing as long as the trigger is depressed). They can, however, be configured to fire dozens of rounds rapidly and to be easy to reload.

The legislators faced enormous opposition, which they softened by putting in a sunset clause. The law was then allowed to lapse in 2004 by the George W. Bush administration. While it is true that some criteria used for assault weapons in the 1994 law contained loopholes that were sometimes sidestepped by manufacturers, it is not true that no tighter legislation could be crafted now.

Assault weapons are what make mass killings possible. They are not used in most of the roughly 14,000 murders committed every year in the United States. But they are used in half of all mass killings. Mass killings are traumatizing for the nation in a way that most murders are not. Individual murders are often not reported in detail on television news, and most are not national news.

A mass killing of 20 children, such as at Sandy Hook Elementary on Dec. 14, 2012, (along with six adult staff members), each riddled with multiple bullet holes, produces powerful social anxieties. This is a kind of evil worth avoiding, as Australia’s Parliament decided in the 1990s, and it can be avoided, as Australia has demonstrated. No hunters would be inconvenienced by an assault weapons ban, and if they are firing 100 rounds in rapid succession at deer, they should be publicly shamed.

The shootings in Chattanooga underline, however, that a society flooded with assault weapons is at special security risk. All U.S. military personnel and police are potential targets. NRA apologists argue that deaths of police by assault weapons did not decline after the 1994 ban, but they neglect to say that the ban applied only to new rifles and left the previously purchased stock of such weapons in place. Moreover, unscrupulous manufacturers exploited loopholes to go on making and selling some of the models. Therefore, the ban would have had a significant impact on police deaths only over many years, and only once the legislative language was tightened. For all we know, the experiment was on the verge of having such an impact when it was abruptly ended.Assault weapons are already a significant source of mayhem. Typically every year, between 1 in 6 and 1 in 5 American police officers who die in felonious attacks are killed by assault weapons. Many are killed by members of drug gangs. Mexican gangs, moreover, have purchased thousands of semi-automatic rifles with expanded ammunition drums from the United States so as to continue a war on Mexican law enforcement.

The Mexican government estimated that 80 percent of assault weapons in that country came from north of the border. The easy availability of such powerful, lethal guns has contributed to a drastic deterioration of American security on its southern border that has also had an impact on trade and employment, in addition to enabling more successful drug smuggling into the U.S.

The Charleston shooting shows another danger of rapid-fire weaponry. Lone wolf extremists can use such weapons to powerful effect. Moreover, such extremists increasingly favor what are called “soft targets” — that is, shooting down large numbers of ordinary people in an attempt to push politics in a particular direction. Dylann Roof, charged in the June 17 killings of nine churchgoers, including a state senator, in Charleston, is alleged to have said he wanted to start a race war. His semi-automatic Glock pistol was deadly enough, and the fact that a flawed FBI background check allowed him to walk into a store and buy one is scary.

Roof used the semi-automatic feature to shoot each victim multiple times and make sure he or she was dead. If he’d had access only to an ordinary revolver, he could not have pulled off the same crime and might well have been overtaken before he could have shot as many people, and more might have lived. But before the crime, Roof had been seeking an assault rifle, and had he gotten hold of one, his aspirations to mass murder and a race war would have been easy to accomplish.

In a day of hard-to-track, hard-to-predict lone wolf terrorism and a time when hitting soft targets is increasingly a terrorist’s tactic of choice, to have a society flooded with easily acquired assault weapons is like setting the timer to a block of C4 explosives in one’s own living room. Add that to the vulnerability of police and stateside troops resulting from this civilian arsenal, and you have a very dangerous terrorism threat.

If American legislators can’t be moved to stop the madness for the right reasons — the halt of mass killings of elementary school students, college undergraduates and theatergoers that not only destroy lives but also damage the national psyche — they should act before it is too late on national security grounds.

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