It's called a varmint cartridge," says Danny O. Coulson, a former FBI Hostage Rescue Team commander, "because it's used for killing foxes, coyotes, rabbits and prairie dogs." Last week someone in the Washington area was using it, with chilling efficacy, to hunt people. After seven sniper-style killings, Montgomery County, Md., police and federal investigators linked five of the murders to the same gun, firing a lightweight .223 round.
Unfortunately, the common .223 could have been used with a long list of rifles or assault weapons. But the bullet offered clues. Because the police did not find casings at most of the...