IN THE MIDDLE of a narrow, darkened room, Officer Larry Hagler sat hunched over a microscope, peering at a mangled slug that had been removed from a homicide victim's spine.

He had spent hours staring at the bullet, like it was a magic puzzle with a surprising hidden image - and, in a way, it was.

Hagler, a member of the Police Department's Firearms Identification Unit, said he was studying the "hills and valleys," the minuscule markings that are left on a bullet when it's fired from a gun.

If he's lucky, after staring at the bullet for a couple of hours or a couple of days, he'd notice that the hills and valleys on the malformed slug match the pattern on a bullet from a different crime scene, thus helping detectives resolve an unsolved case.

On TV shows, that kind of forensic analysis usually takes mere seconds, and leads to a pulse-pounding arrest before the next commercial break.

In the real world, the work done by Hagler and the 19 other members of the FIU is time-consuming and mentally exhausting - but plays a vital role in fighting crime.

"People always say to me, 'What, you guys can't just put a gun in a machine, and have it tell you where the bad guy is right now?' " said Lt. Vincent Testa, the commander of the FIU.

"Well, what machine is that? The coffee machine or the copier?"

Heavy load

The FIU is on the receiving end of every gun, shell casing and bullet fragment that cops find at crime scenes.

The volume is mind-numbing. Last year, Testa said, the unit processed more than 5,000 firearms and more than 1,000 bullet fragments.

"Let's say you have 40 shell casings at one scene. For all intents and purposes, they all have to be examined, processed and catalogued," he said.

On an average Monday morning, the FIU might have 90 or 100 new weapons to process - a weekend's worth of items found in drug raids and shootings.

Testa divides the weapons among his 20 forensic examiners, who must test-fire all of the guns into a water tank to ensure that they work.

"For a person to be charged with violating the Uniform Firearms Act, we have to first make sure the weapons are actually capable of being fired," Testa said.

Examiners run the guns' serial numbers through a database in an attempt to paint a history of each weapon.

"A lot of the guns we get have the serial numbers filed off. But we have chemicals that can bring back the numbers on about 80 percent of them," Testa said.

"Then we can look back and figure out if the gun came from a straw purchaser."

Despite the heavy workload and relative anonymity, morale has been high since Police Commissioner Charles Ramsey added 10 cops and two civilian examiners last year to a unit that had just eight members, Testa said.

With the additional staffing, the unit whittled a backlog of cases that had grown to more than 6,000 down to about 1,500, he says.

"They work incredibly hard," said Chief Inspector Evelyn Heath, head of the Forensics Bureau.

"It might seem like they have a great job, because they're not working the streets, but the work they do isn't easy. They do the investigations on the guns that kill our cops."

'Gangster Wall'

Some of the guns handled by the FIU end up in an evidence room at City Hall.

Police wait until five years after a case is solved before melting down many of the weapons.

Firearms that were used in homicides are never destroyed, Testa noted, because "40 percent of all murder cases are re-examined at some point."

Other weapons remain at FIU's headquarters, in the basement of the department's Forensic Science Building at 8th and Poplar streets in North Philadelphia.

In 2001, the city renovated and redesigned the massive brick building, which generations ago was home to the Mary Channing Wister School.

The facility also serves as headquarters for the Crime Scene Unit and DNA laboratories.

Among Testa's favorite attributes of the building is a white-walled, climate-controlled room that contains 1,200 firearms that represent nearly every make, model and era imaginable.

Some examiners rely on the weapons on display as reference points as they study a new case.

Others venture inside just to gawk.

On one wall hangs a pair of dueling pistols from the 1600s, and diagrams of a pump-action shotgun.

On another wall is an array of items - canes, flashlights and pocket knives - that clever crooks fashioned into one-shot pistols.

"We show that kind of stuff to rookie cops so they're aware of what they can run into out there," Testa said.

Testa's gaze drifted to the "Gangster Wall," which holds Mac 10s, Uzis and guns with homemade silencers - the types of weapons that have been glorified in countless movies and shoot-'em-up TV shows.

Another area of the room features weapons from World War II, including rifles, a cannon and several models of German lugers, items that have been donated by people who found the weapons in their grandparents' attic.

"It's a pretty cool room," he said, grinning.

Bullet study

Although the FIU's collection of firearms can prompt endless "oohs" and "ahhs," the work examiners do with the bullets fired by those guns is equally impressive.

Photos of bullets and shell casings that are processed by the FIU are scanned into the Integrated Ballistic Identification System (IBIS), a computerized catalog of projectiles that have been examined by police, FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives, Testa said.

When a new projectile is scanned into IBIS, the image is shown on a split-screen alongside an ever-changing photo array of other bullets and shell casings of the same caliber.

An IBIS technician, like Michelle Weaver, watches the screen intently for hours on end, hoping to spot a match.

When Weaver finds a match, she notifies guys like Hagler, who study the bullets' hills and valleys on a comparison microscope to determine if the match is legitimate.

Even if he agrees, a second examiner is called in to study the projectiles.

"Every case has peer review," Testa said. "One hundred percent agreement is needed for us to say definitively that we have a match.

"The thing is, our evidence can get you acquitted, or it can get you convicted. We don't think about the guilt of the individual. We just make sure that we get it right."

The FIU is ranked No. 2 nationally for the number of confirmed matches they've found through IBIS, Testa added.

Because of the popularity of shows like "CSI," judges and juries often expect detailed forensic evidence to be a part of criminal court cases, Testa said.

Defense attorneys sometimes bring in their own forensic experts to question the FIU's work.

Testa said FIU investigators have similarly had to step up their game, employing every effort imaginable - like test-firing weapons through pillows, sheet rock and even pig carcasses - to prove or disprove a case.

Potential examiners, in addition to completing a two-year apprenticeship to join the unit, are even told to study photography and chemistry in college, he said.

"I sometimes think 'CSI' is the worst thing that ever happened to us," Testa said.

"But who knows, maybe not. Maybe people actually appreciate the work we do."