It blew away his buttocks, it blew away the back of his legs. It tore off his left arm above the elbow and sent it flying out ahead of him into the road. As the force of the thing hurled him through the air, Fred Downs stared at two glistening white rods imbedded in a piece of bloody meat. They weren't rods at all, they were the forearm bones of what a second ago had been his other arm.

U.S. Army Lt. Fred Downs, of Kingman, Ind., 23 years old, had stepped on a Bouncing Betty. The mine was about the size of a fruit juice can. He had lost one arm entirely, and another had been shattered. In a billionth of a second the world was different.

His buddy Spagg reached him first. Downs, sinking into shock, saw the shock in his friend's eyes.

"Is there anything I can do for you, sir?" choked Spagg.

"Yeah," the lieutenant ground out. "Run over there and pick up my left arm and bring it back to me. I don't want to leave it in this stinking village so the dinks can see what they did to me."

This isn't a Mike Hammer movie, though Mickey Spillane would probably give a lot to have dreamed up such a small, precise tableau of horror. This scene actually happened, 14 years ago, in a green upjungle nightmare called Vietnam. The person it happened to is 38 now and miraculously rehabilitated. Awhile ago he wrote a book about Vietnam called "The Killing Zone." Now, though, Fred Downs, government worker, is appearing in another medium. He has a bit part in the movie remake of Spillane's "I, the Jury" (which opened in Washington yesterday). The movie moguls, you see, needed a man with a fake limb. Downs qualified.

Fred Downs, GS-15, has an intercom phone and cowboy boots and a pleasantly bureaucratic office deep in the cavernous interiors of the Veterans Administration. There are computer printouts taped to his wall, and there is a wondrous harness of plastic and steel fitted onto a stump just below his left shoulder. Downs manipulates this marvel of engineering by expanding and contracting his shoulder blades, which creates tension on the harness, which in turn pulls a cable that connects to a shiny pincer hook.

He can make the hook pick up Styrofoam cups. He can make it grip the steering wheels of cars and the control sticks of old Mooney airplanes. In a drawer at home there is a cosmetic hand that fits onto the end of this prosthetic arm, but Downs thinks it looks too fakey. He likes the hook better. It isn't a macho thing.

He is utterly unself-conscious. (This morning he is in a short-sleeve white shirt, the better for you to get the full view.) "Little kids are fascinated," he says matter-of-factly. "They want to touch it, they want to know how you got it. If they're real little, I tell them I got blown up by a giant firecracker. People just want to satisfy their curiosity."

Every day, when his job at the VA is done, Fred Downs runs home to South Fairlington. It's a seven-mile run. In his office he laces on his tennis shoes and then shrugs out of his harness and puts his arm into a backpack along with his work garb. He changes into his running clothes. He does his warm-up exercises and then takes the elevator to the street. A one-arm man back from the other side of Nam begins a seven-mile run. "I have really lousy circulation in my legs," he says. "I have to keep exercising them to keep the poisons out."

Fred Downs, bit player in the movies, is the VA's director of Prosthetics and Sensory Aids Services. He is not your central-casting bureaucrat. He talks you to death -- in a cackle somewhere between Herb Shriner and David Letterman. He has the Hoosier's ribald, detached view of the world. Not an awful lot tilts him, you'd wager. The man is chock-full of corn. Call him up and ask how he is today and he might answer, "Fine as frog spit."

He was working in his office on Vermont Avenue one afternoon a year and a half ago when the phone rang. It was a friend from Vietnam named Hugh McLure who had flown medevac choppers. Dustoffs, they called them. Hey, how'd you like to be in the movies, Fred. C'mon, Downs said. No, really, McLure said. He said he knew a guy who's looking for a one-armed guy who would be willing to appear naked from the waist up in a Mike Hammer movie.

"Look, I'm not going to be in a porno or play a vet who's wacko," Downs said.

Next came the call from the Coast. This is still the land of beans and dreams. You're doing your job and all of a sudden Hollywood is on the line. We need a one-armed guy for a scene at the top of the movie, a voice said. The scene only lasts a couple of minutes but all important. You're yourself, the voice said. Okay, Downs said: "I'm 5 foot 9 1/2. I got a medium build and a good personality and I'm devilishly good looking."

"Oh. That's too bad. We were looking for somebody who looks a little rough."

But life has a way of working out and pretty soon Fred Downs was sitting next to a man from Hollywood in a Rosslyn hotel. The guy was bald, kept knocking back martinis, had a tan that wouldn't quit and a shirt split to the navel. Lots and lots of gold chains and little phallic symbols everywhere you looked. But he was a swell guy, it turned out. "I think you're gonna do, Freddy," he said. Already the people from the Coast had started calling him Freddy.

He got the job and went to New York for shooting. "Please put in that I took vacation time from my job." There were chauffeurs and gofers and a couple hundred bucks a day with all expenses paid. Mostly you sat around on the steps of some creepy old building on the West Side, waiting for your scene. The dream lasted about a week. At night they'd look at the rushes. He celebrated his 37th birthday on the set. He even had his own stand-in. "You couldn't expect me to lie on the floor. I was an Actor."

Alas, the actor only had one line. For the part in the body bag, the director kept coming over and shouting through the plastic, "You okay in there, Freddy? Hey, Freddy, can we get you some more ice?"

The key part of his scene was getting blown away by a sadistic femme fatale from a sex clinic. Downs figured he had been blown away once before; this one would be a cinch. They used a prosthetic gel to make the hole in his stomach, flesh-toned it, glued on fake hair. Somebody named Carl was the makeup man. Genius. He molded the hole perfectly to Downs' stomach. The blood, Downs remembers, was food coloring, Kayo syrup and water. He didn't suffer flashbacks.

But was it hard to come back to the bowels of government?

"No, because my intrinsic thing is that I believe in order. I like government. I believe the individual can make a difference. As a romantic I couldn't get enough of that movie, but as a realist I knew it was all going to go poof in a week."

Around the right wrist of Fred Downs' arm, the real one that was sewn back together with parts of his legs, elbow and stomach, there is a bracelet with this imprinted on it: "1/11/68. 0745. LT. FREDERICK DOWNS JR. Victory is never complete."

That was the moment in Vietnam he stepped on Bouncing Betty.