Although Friday's massacre at Sandy Hook Elementary School has prompted renewed calls for reinstating the federal "assault weapon" ban, we know for a fact that such a law would not have stopped Adam Lanza or made his attack less deadly, because it didn't. The rifle he used, a .223-caliber Bushmaster M4 carbine, was legal under Connecticut's "assault weapon" ban, which is similar to the federal law that expired in 2004. Both laws, in addition to listing specifically prohibited models, cover semiautomatic rifles that accept detachable magazines and have at least two out of five features: 1) a folding or telescoping stock, 2) a pistol grip, 3) a bayonet mount, 4) a grenade launcher, and 5) a flash suppressor or threaded barrel designed to accommodate a flash suppressor. The configuration of the rifle used by Lanza, which his mother legally purchased and possessed in Connecticut, evidently was not covered by that definition. A Huffington Post article blames "loopholes": 

Some gun-control advocates have voiced concerns that companies like Bushmaster are able to exploit loopholes in state versions of the 1994 Federal Assault Weapons Ban, which outlawed the sale of 19 types of military-style assault rifles. At the time, weapons like the Uzi and the Mac 10 were widely used in inner-city gang crime. The federal law lapsed in 2004 but many states have passed their own bans in the interim.

The loopholes in the law are evident in Connecticut, which has a state assault weapons ban modeled closely on the lapsed 1994 federal ban, said Ladd Everitt, spokesman for the Coalition to Stop Gun Violence. The Connecticut law bans certain assault weapon models that combine multiple features, such as a pistol grip and a barrel shroud, which increase a gun's lethality.

“What’s happened is the gun makers figured out how to take their existing assault weapon models, modify them just a bit and bam, they can sell,” Everitt said. "If it has a grenade launcher and barrel shroud, you take the grenade launcher off and then you can sell it. If it has a pistol grip and some other banned feature, you drop one of them.”

What may be needed is a longer list of banned or forbidden gun models and single gun features that would render more assault weapons illegal to sell, Everitt said.

But the term assault weapon was invented by the anti-gun lobby as a way of blurring the distinction between military-style semiautomatics, which fire once per trigger pull, and selective-fire assault rifles, which can be set to fire continuously (a distinction that President Obama, who wants to bring back the "assault weapon" ban, either does not grasp or deliberately obscures). Since that neologism has no meaning independent of the laws that define it, there is little sense in saying the laws should be changed to cover more "assault weapons." Guns are not "assault weapons" until legislators arbitrarily decide they are.

Speaking of which, a bill introduced by Rep. Carolyn McCarthy (D-N.Y.) takes the approach suggested by Ladd Everitt, making any one of those five features, plus a detachable magazine, enough to qualify a rifle as an "assault weapon." Her legislation also prohibits the transfer of "any assault weapon with a large capacity ammunition feeding device," but (like the earlier ban) it apparently does not apply to firearms produced before the bill takes effect. If it did, it would transform millions of law-abiding Americans into felons overnight. But since it doesn't, it would not have much of an impact on the types of guns available to murderous lunatics, even assuming that any of the features singled out by McCarthy makes a practical difference in mass shootings.

New York Times reporter Erica Goode notes that the rifle used by Lanza is modeled after the Colt AR-15, a civilian version of the M16, and that guns of this type are "the most popular rifle in America," with an estimated 3.3 million to 3.5 million sold since 1986. According to a 2011 survey of gun dealers by the National Shooting Sports Foundation, 49 percent of AR-15-style rifles were purchased for target shooting, 23 percent for hunting, and 28 percent for personal protection. These supposed "weapons of war" clearly have legitimate uses, which far outnumber criminal ones. Goode notes that rifles of any kind were used in less than 3 percent of homicides last year.