But an Internet search returned very few hits. None of the big manufacturers like Winchester, Remington or Federal offered it and none of the retail ammo sellers stocked the bullet. Sachs, however, found some references to the mysterious round’s existence in some obscure shooting forums and learned that an arms company in New Jersey, Walker Defense Systems, might be the maker. Its website revealed that, though Walker didn’t make rifles, it manufactured a plastic-tipped spitzer .420 boattail.

Sachs looked at Rhyme. “They only sell to the army, police…and the federal government.”

The first goal was satisfied, the ID of the bullet. Now the team turned to finding the type of weapon that had fired it.

“First of all,” Rhyme asked, “what kind of action was it? Bolt, semiauto, three-shot burst, full auto? Sachs, what do you think?”

“Snipers never use full auto or bursts—too hard to compensate for repeated recoil over distance. If it was bolt-action, he wouldn’t have fired three rounds. If the first one missed, he’d’ve alerted the target, who’d go to cover. Semiauto, I’d vote.”

Sellitto said, “Can’t be that hard to find. There’s gotta be only one or two kinds of guns in the world that’ll fire a slug like that. It’s pretty unique.”

“Pretty unique,” Rhyme blurted, with a frosting of sarcasm. “Just like being sort of pregnant.”

“Linc,” Sellitto replied cheerfully, “you ever think about teaching grade school? I’m sure the kids’d love ya.”

Sellitto was right substantively, though, Rhyme knew. The rarer the bullet, the fewer the types of guns that will fire it. This would make it easier to identify the rifle and therefore easier to trace it to Barry Shales.

The two characteristics of a bullet that link it to the weapon that fired it are caliber, which they now knew, and rifling marks.

All modern firearms barrels have spiral troughs cut into them to make the bullet rotate and thus move more accurately to the target. This is known as rifling (even though it applies to pistols too). Gun manufacturers make these troughs—called lands (the raised part) and grooves—in various configurations, depending on the type of gun, the bullet it’s intended to shoot and its purpose. The twist, as it’s called, might spin the bullet clockwise or counter, and will spin it faster or slower depending on how many times the slug revolves in the barrel.

A look at the slug revealed that Barry Shales’s gun spun the slug counterclockwise, once every ten inches.

This was unusual, Rhyme knew; spirals are generally tighter, with the ratio of 1:7 or 1:8.

“Means it’s a long barrel, right?” Rhyme asked Cooper.

“Yep. Very long. Odd.”

Given the rare caliber and rifling, it would normally be easy to isolate brands of semiautomatic rifles that produced characteristics like that. Ballistics databases correlate all this information and a simple computer search returns the results in seconds.