KARMA, Iraq, Oct. 30 — Petty Officer Third Class Dustin E. Kirby clutched the injured marine’s empty helmet. His hands were coated in blood. Sweat ran down his face, which he was trying to keep straight but kept twisting into a snarl.

He held up the helmet and flipped it, exposing the inside. It was lined with blood and splinters of bone.

“The round hit him,” he said, pausing to point at a tiny hole that aligned roughly with a man’s temple. “Right here.”

Petty Officer Kirby, 22, is a Navy corpsman, the trauma medic assigned to Second Mobile Assault Platoon of Weapons Company, Second Battalion, Eighth Marines. Everyone calls him Doc. He had just finished treating a marine who had been shot by an Iraqi sniper.

“It was 7.62 millimeter,” he continued. “Armor piercing.”

He reached into his pocket and retrieved the bullet, which he had found. “The impact with the Kevlar stopped most of it,” he said. “But it tore through, hit his head, went through and came out.”

He put the bullet in his breast pocket, to give to an intelligence team later. Sweat kept rolling off his face, mixed with tears. His voice was almost cracking, but he managed to control it and keep it deep. “When I got there, there wasn’t much I could do,” he said.

Then he nodded. He seemed to be talking to himself. “I kept him breathing,” he said.

He looked at Lance Cpl. Matias Tafoya, his driver, and raised his voice. It was almost a shout. “When I told you that I do not let people die on me, I meant it,” he said. “I meant it.”

He scanned the Iraqi houses, perhaps 150 yards away, on the other side of a fetid green canal. Marines were all around, pressed to the ground, peering from behind machine-gun turrets or bracing against their armored vehicles, aiming rifles at where they thought the sniper was.

The sniper had made a single shot just as the marines were leaving a rural settlement on the western edge of Karma, a city near Falluja in Anbar Province.

The marines had been searching several houses on this side of the canal, where they found five Kalashnikov assault rifles and bomb components, and were getting back into their vehicles when everyone heard the shot. It was a single loud crack.

Image Petty Officer Third Class Dustin E. Kirby, at far left in foreground, and members of his platoon prayed for Lance Cpl. Colin Smith, who was wounded by a sniper. Credit... Joao Silva for The New York Times

No one was precisely sure where it had come from. Everyone knew precisely where it hit. It struck a marine who was peering out of the first vehicle’s gun turret. He collapsed.

Petty Officer Kirby rushed to him and found him breathing. He bandaged the marine’s head as the vehicle lurched away. Soon he helped load the wounded marine into a helicopter, which touched down beside the convoy within 12 minutes of the shot.

Once the helicopter lifted away, he ran back to his vehicle, ready to treat anyone else. He was thinking about the marine he had already treated.

“If I had gone with him,” he said, and glanced to where the helicopter had flown away, over the line of date palms at the end of a field. His voice softened. “But I’m not with him,” he said.

He turned, faced a reporter and spoke loudly again. “In situations and times like this, I am bound to start yelling and shouting furiously,” he said. “Don’t think I am losing my mind.”

He held his bloody hands before his face, to examine them. They were shaking. He made fists so tight his veins bulged. His forearms started to bounce.

“His name was Lance Cpl. Colin Smith,” he said. “He said a prayer today right before we came out, too.”

“Every time before we go out, we say a prayer,” he said. “It is a prayer for serenity. It says a lot about things that do pertain to us in this kind of environment.”

The only sounds were Doc’s voice and the vehicle’s engine thrumming.

He recited the prayer. There was a few moments of silence. “It’s a platoon kind of thing, if you know what I mean,” he said.

He listened to his radio headset and looked at Lance Corporal Tafoya, relaying word of the marines’ movements. “Right now the grunts are performing a hard hit on a house,” he said. He turned back to the subject of Lance Corporal Smith, 19.