An 11-year-old Annandale boy was shot to death yesterday by his ten-year-old playmate, after the youngsters somehow took a handgun belonging to the younger boy's father from a locked drawer and managed to load it.

Bruce Tiemens, who turned 11 on Friday and had spent the last few days excitedly planning his birthday party for this afternoon, was killed by a single shot to the head from the .45-caliber automatic. Police tentatively have ruled the shooting accidental, and the family of the younger boy has consulted a psychiatrist to lessen the boy's trauma of having shot his classmate and friend.

The gunshot, at 1:30 p.m., devastated two families who lived nearby in an affluent suburban neighborhood. It is the type of area where expensive cars sit in front of large, brick colonials, where violence is a remote concept.

The father of the boy who fired the pistol said he had been at a swim meet when the shooting occurred.

"There are so many ifs," he said. "If I had taken the boy with me, even though he had already swum in the morning. You can kill yourself with ifs.

"He'll have to live with this for a lifetime. My son doesn't want to go back to school because he's afraid people will call him a murderer. You know that kids that age live by their peers."

Myron Tiemens of 8311 Kay Ct., father of the slain youth, dashed to the neighbor's house as soon as the surviving boy ran to him for help. He said he immediately felt the artery in his son's neck for a pulse, but found none.

"We're devastated," said Tiemens, 50, a division director for the Environmental Protection Agency. "He was the light of our life."

Bruce was tall for his age, a thin, sandy-haired boy with blue eyes. He was the youngest of six brothers and sisters -- the youngest by seven years.

"He was all of our baby," said his sister Sandy, 20. "We took care of him from diapers on."

The family assembled in their comfortable living room hours after the tragedy, awkwardly stumbling with the past tense as they talked of Bruce. He was an impish, cheerful boy who had once been a slow learner plagued with physical coordination problems. With the help of his family, he had advanced to scholastic and athletic achievements. He had done well this year in the local soccer league, and had just won a spot on the school basketball team.

"He always appreciated our time with him," said his 22-year-old brother, Ron. "I remember that I covered his textbook with a brown paper bag and really did a rotten job on it, and he was so grateful. He was never a spoiled kid."

The Tiemens household was a center for Bruce's playmates in the area. They always were welcome, and the grass in the side yard is torn up from their frequent soccer games.

Bruce was a sports fanatic. His favorite athlete was Pele, the Brazilian soccer star. His sister, Karen, was planning a visit to Brazil later this year, and Bruce had made her promise to visit Pele's home stadium. Bruce had done a report on Pele for school. His mother, Anita, 47, said last night that she would like to have the paper back.

A methodical child, Bruce had written detailed plans for the birthday party that was scheduled for today. He had divided his planning document -- executed in pencil on a torn piece of computer paper -- into five headings: Food, Prizes, Games, People Going, and Time and Place.

"We've read stories of this happening to other families," Myron Tiemens said. "It just seems so incredible that it's happening to us. It will just be another statistic."

The day began for both families as a typical Saturday in the suburbs, the kind of day for a father to watch his children in a swim meet, for young boys to play games in the living room, for adults to don their bluejeans and sportshirts. It ended with two families searching for answers.

"I was raised with guns, to understand them and to know their use," said the father of the 10-year-old boy. He said he did not understand how his son managed to load the gun and fire it. The gun was kept in a locked drawer, but the father said it was no family secret where the key was kept.

"If his death can rouse just a few more people to rally against handguns, I would be grateful," Myron Tiemens said. "These handguns have somehow got to be put out of kids' hands."

Added Bruce's sister, Karen, 25: "It just seems incredible so many more kids are killed by handguns than rapists or burglars get killed."

The family of the slain boy requested that any memorials be sent to them to donate to handgun control organizations.