U.S. Army observers watched as the white pickup truck sped from house to house in Fallujah on Wednesday morning, stopping more than 20 times to drop off armfuls of rocket-propelled grenades. Every few stops, the driver threw dirt on the roof of the cab, apparently in an attempt to disguise his vehicle.

A few miles away, soldiers in the 1st Infantry Division's 1st Platoon, Alpha Battery trained their M109A6 Paladin, a self-propelled 155mm howitzer, on the truck. Minutes later, a shell shot out of the cannon, whizzed across the sky and landed next to the truck with a massive boom, shooting off shrapnel.

In the resulting cloud of dust and fire, the observers could not tell whether the truck had been hit directly. But even if the driver got away, said Staff Sgt. Shawn Zawistowski, a member of the 1st Infantry's Task Force 2-2, "I guarantee we made him think twice."

Powerful artillery pieces such as the Paladin deserve much of the credit for the ease and speed with which the U.S. military has been able to take control of most of Fallujah, according to American soldiers who have been sweeping through the city over the past two days.

Before ground troops entered Fallujah on Monday night, warplanes pounded insurgent targets with bombs; mobile artillery batteries followed with cannon and mortar fire. The effect was significant, according to military commanders and soldiers inside the city.

"It's made everybody get out of town," said Zawistowski, 30, of Cleveland.

Alpha Battery's two artillery pieces have fired more than 300 rounds in the first three days of the battle. The Marines' Mike Battery 414, which has six big guns at the same military outpost, has launched more than 500 rounds.

"A lot of times people don't take what we do seriously because we're not out there getting shot at. But we take the battle really seriously," said Alpha Battery's Capt. Jeff Fuller, 25, of Coshocton, Ohio. "We can save those guys' lives out there."

"We do this to support our troops on the ground to move faster," said Marine Capt. Kirk Parsons of Atlanta. "For the last few days, we gave a lot of support to hit a lot of strong points and areas where the insurgents try to set up their fire."

The artillery batteries zero in on targets based on grid coordinates provided by field observers and air surveillance. At the 1st Infantry's fire direction center, a tent attached to a mobile command center near the artillery guns, soldiers monitored radio traffic Wednesday and waited for instructions to shoot. Early in the day, they fired at a mosque where at least 20 insurgents were reported to be holed up with a weapons cache.

The Paladin fires rocket-assisted shells that can travel up to 22 miles and regular shells that can cover 13 miles. The shells typically strike within about five yards of their target and are likely to kill anyone within 55 yards of the point of impact.

"When we get targets, we take them out," said Sgt. 1st Class Johnny Dotson, 38, of Brownsville, Tenn., who said he joined the Army 19 years ago to be an artilleryman.

As he talked, a boom, followed by a sharp crack, pierced the cloudless sky above the field where the Army's guns were positioned between wide mounds of dirt.

"That was the Marines," said Dotson, who is known to his comrades as "Smoke."

The force of the blast knocked over a pile of empty canisters near an artillery piece nicknamed "Betsy" by its crew chief, Brian Blakey, 31, of Russellville, Ky.

Sgt. Fladymir Napoleon, 25, of Asbury Park, N.J., restacked the canisters in piles of three.

"It's a great thing blowing stuff up," said Napoleon, who has been in Iraq for nine months. "We're getting the city free, back to democracy. I'm feeling pretty good. We play a big role in the battle."

Inside the Paladin's turret, where the chief and two crew members load the big gun, Blakey rubbed his hand across a 155mm round sitting in the chamber. "Three of these," he said, patting the round, "and I can take out a whole building."

Blakey said he tries not to think about what the shell hits -- humans or structures.

"I just look at it like we've got people out there, too," he said. "If we don't get rid of the target, one of our guys could get killed. As long as no civilians are out there, we're doing all right."

Blakey said an Army recruiter persuaded him to be an artilleryman by describing the Paladin. "He told me it was the biggest gun in the military, and I said, 'I'll take it,' " he recalled.

Blakey said he senses the importance of artillery's role in the Fallujah operation.

Iraqi authorities "want to have elections in January," he said. "In order for them to have democracy, or whatever you want to call it, whatever, if we take care of Fallujah, the guys who come in next, they won't go through the same things we did. It's worse now than during the war."

At the other gun a short distance away, Spec. John Kennedy, 26, of Dallas, asked Dotson about the rounds his crew had fired that morning. "What were we shooting at?" he asked. "Did we get it?"

Yes, Dotson told him. They hit the mosque. Twenty confirmed killed.

"We really get no glory," said Staff Sgt. Jason Moye, 25, of Phoenix.

Special correspondent Omar Fekeiki contributed to this report.

Sgt. Fladymir Napoleon, 25, a member of an Army artillery unit, is surrounded by empty canon casings and artillery rounds near the city of Fallujah.2nd Lt. David Martin, 23, right, reports to a tent near Fallujah where soldiers monitor radio traffic and wait for instructions to fire artillery.