Warning about "weapons designed for the theater of war," President Obama on Wednesday called for immediate action on a new Federal Assault Weapons Ban. He said that "more of our fellow Americans might still be alive" if the original assault weapons ban, passed in 1994, had not expired in 2004. Last month, in the wake of the horrific shooting at Sandy Hook Elementary in Newtown, Conn., Sen. Dianne Feinstein (D., Calif.) promised to introduce an updated version of the ban. She too warned of the threat posed by "military weapons."

After the nightmare of Newtown, their concern is understandable. Yet despite being at the center of the gun-control debate for decades, neither President Obama nor Ms. Feinstein (the author of the 1994 legislation) seems to understand the leading research on the effects of the Federal Assault Weapons Ban. In addition, they continue to mislabel the weapons they seek to ban.

Ms. Feinstein points to two studies by criminology professors Chris Koper and Jeff Roth for the National Institute of Justice to back up her contention that the ban reduced crime. She claims that their first study in 1997 showed that the ban decreased "total gun murders." In fact, the authors wrote: "the evidence is not strong enough for us to conclude that there was any meaningful effect (i.e., that the effect was different from zero)."

Messrs. Koper and Roth suggested that after the ban had been in effect for more years it might be possible to find a benefit. Seven years later, in 2004, they published a follow-up study for the National Institute of Justice with fellow criminologist Dan Woods that concluded, "we cannot clearly credit the ban with any of the nation's recent drop in gun violence. And, indeed, there has been no discernible reduction in the lethality and injuriousness of gun violence."

Moreover, none of the weapons banned under the 1994 legislation or the updated version are "military" weapons. The killer in Newtown used a Bushmaster .223. This weapon bears a cosmetic resemblance to the M-16, which has been used by the U.S. military since the Vietnam War. The call has frequently been made that there is "no reason" for such "military-style weapons" to be available to civilians.