Dr. Colin Poole, head of surgery at Ringriket Hospital in Honefoss northwest of Oslo, told The Associated Press that surgeons treating 16 gunshot victims have recovered only tiny fragments on bullets from victims' bodies, adding that the exit wounds were unusually small and weak.
"These bullets more or less exploded inside the body. All the energy of the bullets was deposited inside the tissue," Poole said. "They inflicted internal damage that's absolutely horrible."
The 32-year-old Norwegian gunman, Anders Behring Breivik, opened fire on the island, killing at least 83 people. Seven others were killed in a bomb blast in Oslo hours before the shooting. Breivik's lawyer claims the suspect acted alone in both attacks.
Ballistics experts say dum-dum bullets are lighter in weight and can be fired with greater accuracy over varying distances. They commonly are used by air marshals and hunters of small animals.
Poole, a surgeon for 26 years at the hospital, said the bullets were "hyper-fragmentable" and produced confusing pictures on X-rays.
"It's caused us all kinds of extra problems in dealing with the wounds they cause, with very strange trajectories," he said. "The effect they cause inside the body is like a thousand pin pricks."