In the 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 Swannack Jr., who recently retired after commanding the 82nd Airborne Division in Iraq.

In the 1990s, fiddy-cals and crates of .50-caliber 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 Browning during World War I.

The gun alone weighs 84 pounds, not including its 40-pound tripod and heavy brass-jacketed ammunition.

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 drain ammo stockpiles rapidly. 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.