WASHINGTON -- U.S. troops in Iraq are firing .50-caliber machine guns at such a high rate, the Army is scrambling to resupply them with ammunition -- in some cases dusting off crates of World War II machine gun rounds and shipping them off to combat units.

In the dangerous and unanticipated conflict that has intensified in Iraq since the U.S. invasion in March 2003, the gun that grunts call the "fiddy-cal" or "Ma Deuce," after its official designation, M-2, has become a ubiquitous sight mounted on armored Humvees and other heavy vehicles.

Above the staccato crackle and squeak of small arms fire, the fiddy-cal's distinctive "THUMP THUMP THUMP" indicates that its 1.6-ounce bullets, exactly the weight of eight quarters, are going downrange at 2,000 mph. The bullets are said to be able to stop an onrushing car packed with deadly explosives dead in its tracks from a mile away. A .50-cal round can travel four miles, generally not with great accuracy.

At closer ranges, it is so powerful that a round will obliterate a person, penetrate a concrete wall behind him and several houses beyond that, gunners in Iraq have said.

"You can stop a car, definitely penetrate the vehicle to take out the engine -- and the driver," said Army Maj. Gen. Charles H. Swannack Jr., who recently retired after commanding the 82nd Airborne Division in Iraq.

Merely "the noise of it is huge. Intimidating," Swannack said. But it's so powerful, he added, "I would not use it in an area where there's lots of noncombatants."

In the 1990s, fiddy-cals and crates of .50-cal ammunition gathered dust as the Army struggled to shed its heavy image and become lighter, quicker and more high-tech. Fiddy-cals are early Industrial Age artifacts, invented by John Moses Browning during World War I. Browning's 1919 drawings specified machined steel plates and rivets; today's manufacturers haven't monkeyed with his basic design. The gun alone weighs a bone-crushing 84 pounds, not including its 40-pound tripod and heavy brass-jacketed ammunition.

Outmoded or not, when Iraq erupted, the Army and Marines reached back for the .50-cal and its heavy killing power.

Swivel-mounted in the turret of a Humvee, the gun can lay down a heavy steel blizzard, 40 rounds a minute, on grouped insurgents or vehicles, and is often used in convoys or at checkpoints as a last resort to stop suicide car bombers.

Small wonder, then, that the steady increase in .50-cal use began to rapidly drain ammo stockpiles. At the Blue Grass Army Depot in Richmond, Ky., ammunition left over from Desert Storm, Vietnam, Korea and even World War II had been stored in massive concrete bunkers, including some 12 million rounds of .50-cal. They began shipping it off to Iraq.

By the time the war stretched into its second year, the Blue Grass stockpile of .50 cal had shrunk to 4 million rounds.

The Army surged production of new .50-cal ammunition, taking on more than a thousand new workers at its Lake City ammunition plant in Independence, Mo.

"Fifty-cal is crazy," said Bryce Hallowell, spokesman for Alliant Techsystems Inc., the contractor that runs the plant. Four years ago, Lake City was manufacturing about 10 million rounds a year; currently it is producing at an annual rate of 50 million rounds and rising.

Even that five-fold increase hasn't been enough.

At Blue Grass, Darryl Brewer, a combat medic in Vietnam, is chief of logistics for the ammunition depot. Recently, he started pulling out .50 cal. crates marked 1945. He opened some up and peered inside.

"Pristine," Brewer reported. "It's in lead-sealed cans, like sardines. Just like it was made yesterday."

The 1945 ammunition was opened and test rounds fired to check for reliability and accuracy, standard testing done for all aging ammunition. "They find anything wrong, they'll do a suspension," Brewer said, adding with some pride, "Very seldom you see that in a fiddy-cal."

Fifty-cal rounds are linked into belts that are fed from steel ammo boxes into the side of the weapon. At Blue Grass, technicians have to replace the World War II links, using a "delinker-linker" machine so old they had to make parts for it before it would work. The relinked rounds are sealed back in ammo boxes, like sardines, and shipped.

Once grunts open up the boxes in Iraq, "then you start to have deterioration," Brewer said. "Stuff goes pretty fast."

Like other workers at Blue Grass, Brewer, 58, has a personal stake in the war, and the ammo. His son, 1st Lt. William Bryan Brewer, deploys to Iraq in December as a Blackhawk helicopter pilot. Conceivably, suppressive ground fire from .50-cals will force insurgents to keep their heads down as his aircraft passes.

"We got a couple guys with sons over there," Brewer said. "That's why we're kinda particular to make sure this stuff is right when it goes out.

"It could save their lives one day, you never know."