The .357 magnum revolver had a brown handle and blue steel barrel, and the moment Bill McCall saw it in the southern Virginia gun dealer's shop he knew he wanted it. He said he liked the feel of it, the way it fit perfectly in the palm of his hand. The shiny revolver was more powerful than a .38, he said, certainly powerful enough to protect his home with one swift pull of the trigger.

McCall felt it was, as well, the perfect weapon for his wife to use to protect herself and their three children while he was away at work. So, that day in 1979, he bought it and gave it to her, and for two years the .357 magnum sat gleaming atop a shoulder-high chest of drawers in the McCalls' Mount Airy, Md., bedroom with a presence that exuded reassurance.

But at 1:30 p.m. Tuesday, in a manner McCall still cannot fathom, his 3-year-old son was able to find the gun, and the reassurance it once gave turned rapidly to terror. The toddler picked up the weapon and, perhaps thinking it was merely another toy pistol like the others his father had given him, pointed it at his mother.

"Is this what I pull, Mommy?" he asked, and before she could do or say anything, her son pulled the trigger. A bullet exploded from the barrel, passed through his mother's chest and back, and then went through a bedroom window.

Judy McCall, who had been placing photographs into a family room, staggered backward, then stumbled to a nearby telephone, blood starting to stain her gray sleeveless blouse, while her young son, crying, opened a dresser drawer and began frantically searching for a box of Band-Aids. His mother dialed the operator. In the back yard, 13-year-old Mildred McCall, upon hearing the "loud pop" and the shatter of glass, raced inside and up carpeted stairs to the bedroom, where she saw the gun on the floor and the blood streaming from her mother's chest.

Still conscious, Mrs. McCall gave Mildred the phone. Within minutes, Howard County paramedics arrived, followed closely by Maryland State Police. Soon a helicopter landed in a cornfield across the road to take her to Baltimore's Shock Trauma Unit, where she was operated on Tuesday afternoon and where she was listed yesterday in critical but stable condition.

Bill McCall, a Rockville air conditioning specialist, raced home to Mount Airy, arriving shortly after the helicopter departed. Ever since that moment, when he first saw the bloodstains on the bed, the look of confusion, fear and horror in his son's eye, and sniffed the faint odor of gunpowder in the house, he has been cursing himself and the gun "for an accident that was entirely my fault.

"With the help of God, Judy will be all right," he said at his kitchen table, as his son, a blond, blue-eyed boy, bounced up and down on a chair. "Right now, he doesn't know the difference between real and unreal. But if, God forbid, Judy doesn't live, I don't want him to grow up knowing he killed his mother."

McCall brushed his eyes with one hand and wiped long red hair from his face. "Do you need a gun, and why he said. "I've been asking and asking myself that question. I just don't know."

The .357 magnum was one of three guns owned by McCall. The other two are a .30-caliber rifle and a .22-caliber handgun. Those two are kept on a rack in a bedroom closet, he said, adding that he originally wanted the magnum revolver because it was powerful enough "to do the job." It was placed atop the dresser within easy reach should an intruder enter the house.

"There have been break-ins around here, and you read all the time about burglaries. I had a dog here once whose head was beat in; dog was beat to death by somebody," McCall said. "It's not a friendly world. It made sense to have some protection. I'm a hunter. I know about guns. What's hard to take is that I got the .357 specifically to protect the most important people in my life."

Robert Honeycutt, associate editor of the American Rifleman, a publication of the National Rifle Association, said the .357 magnum originally was manufactured in the mid-1930s as a police service revolver, and is still used by many police forces in the country.

"Its cartridge [case] is a tenth of an inch longer than .38s," he said. "You can get more velocity with it and put in more gunpowder. It's used for hunting a lot, but it's also used for house protection. It's powerful enough . . . without being so hard kicking."

The McCall home, yellow and two-storied with bright white shutters, is located on Penn Shop Road two miles west of Mount Airy, just across the Montgomery County line in Howard County, a quiet land of rolling rural pastures, wheat, silos, cornfields and fruit trees.

One night last week, in the predawn hours, the McCalls were awakened by a loud commotion in a cornfield across the street. McCall groggily grabbed the revolver and went to investigate the noise. He found it came from a group of teen-agers, "raising hell," he said, so he returned home and went back to sleep.

He didn't put the revolver back in its accustomed place. In fact, he said, he couldn't remember where he put it. He only knows the his 3-year-old found it.

Last weekend, the McCalls visited friends near the Chesapeake Bay, and spent two days fishing, boating and crabbing. He and his wife took turns snapping photographs. Upon returning, they had the photographs developed. The faces in the pictures were smiling. Hands proudly held fresh-caught fish. The family feasted on crab.

Mrs. McCall was sticking the pictures into the family album when her son, a rambunctious, enthusiastic child, appeared with the .357.

"I've been piecing this together, trying to talk to him gently, you know," McCall said. "He can't understand what's happened. We saw Judy in the hospital and she told us everything. She didn't have time to get it away from him. She heard her son ask her, 'Is this what I pull?' and then the next thing she knew there was the pop."

Maryland state trooper Joseph Marick said Mrs. McCall was able to speak clearly to paramedics when they arrived on the scene, "but she was in awful pain, as you might imagine. The boy was very confused . . . "

Mildred McCall, who was oiling a bicycle in the back yard while her sister Connie, 14, was mowing the lawn, dashed upstairs, spoke to the operator, then called her father. "Daddy," she cried, "[the 3-year-old] just shot Mommy." She then took her brother downstairs and ran to a next-door neighbor who returned with her to the house, emptied the remaining bullets from the revolver and awaited the arrival of paramedics.

"I'd heard gunshots before, so I knew what it was," Mildred McCall said. "But it was just so awful. Shadow [the family's Doberman pinscher] was howling and crying. After I saw Mommy, I went out to try to quiet him down."

Officials of the Frederick Barracks of Maryland State Police said the McCall case was the first shooting incident, aside from three suicides, that they have had to investigate this year. They said McCall requested that they keep the .357 revolver and they complied. "I have no use for it here," McCall told them. The investigation, police said, was completed on the scene and logged as an accidental shooting.

Tuesday night, after stopping to buy flowers at a shop in Mount Airy, McCall and the three children went to visit Mrs. McCall at the shock trauma hospital in Baltimore. After awaiting several hours, they were finally alowed to visit her. Doctors had removed one lung and sewn up the wounds, and she was still drowsy from medication. She opened her eyes, saw her son next to the bed and the first thing she said was, "I love you."

The 3-year-old, according to his father, said the one thing he has always said when he knows he has done something wrong and seeks forgiveness. "Mommy," he whispered, "I promise I won't ever do it again."