NEW YORK (AP) _ Larry Heinemann stood behind a .50-caliber machine gun daily in Vietnam. The work was hard, he said, but the killing was easy.

He returned home bitter and desperate to write about the war. His first novel, ''Close Quarters,'' began as a catharsis; his second, ''Paco's Story,'' just won the National Book Award for fiction.

The 43-year-old author, a combat infantryman during the war, was surprised his work received the honor, which carries a $1,000 prize.

''It's been a very quiet book. So I'm not ungrateful for this, and I do know what it means. ... It's a tremendous honor to be singled out like this,'' he said in an interview Tuesday.

''Both books really deal with how a person develops into a soldier: How spiritless and back breaking the work was - not the killing but the work, because the killing is easy - and the process you have to go through to get there. When people came back from overseas, they were not the same.''

Heinemann certainly wasn't the same person and still feels the effects of jungle warfare in Southeast Asia. His next book, a work of nonfiction, discusses the delayed stress experienced by Vietnam veterans, or post- traumatic stress disorder.

When Heinemann returned to his native Chicago in 1968, after serving with the 25th Division in Dao Tieng and Cu Chi, he was ''absolutely radicalized and extremely bitter - almost more bitter than tongue can tell.'' The bitterness did not destroy him; rather, he channeled it into writing.

''My purpose in writing 'Close Quarters' was to get the event straight in my mind,'' he said. ''You come back from that so spiritless. The one thing you have to do is discover your own humanity first, and then, in order to be a writer, you have to find a way to tell the story - you have to find the language, the form of the story, you have to find a way to get the story out and not flinch.

''One of the things I knew I wanted to tell about the war was just how horribly ugly it was and what mean sons of bitches we were and hateful. I have never hated anyone the way I hated the Vietnamese,'' he said. ''I stood behind a 50-caliber machine gun and I wasn't shy. ... I wrote to tell about the spirit of atrocity; in the field you could do anything.''

In ''Close Quarters,'' he writes about the Army's ''fire base bert'' in which 500 Vietnamese were killed in one night at Soui Cut on Jan. 1, 1968.

''We killed people all night,'' Heinemann said. ''The next morning there was meat all over everything. When I sat down to write about that, I wanted the reader to know absolutely what 500 bodies looked like when they're nothing but meat, and what it smells like and tastes like. Gunpowder leaves a certain pervasive taste in its mouth.''

''Paco's Story,'' published by Farrar, Straus & Giroux, which received the book award Monday, is just as vivid in its retelling of war's carnage. It is a carefully crafted work about the horrors of the Vietnam War as illuminated through a physically and mentally scarred, sole survivor of a massacre at a base camp called Harriette.

Everyone wants to know Paco's story. He listens to all of theirs while he washes dishes at Texas Lunch, a ritual that helps get him through the day and keep away the ghosts that otherwise haunt him.

Heinemann weaves a stark narrative around the massacre, a nauseating rape of a Vietcong woman and Paco's perverted talent for setting booby traps.

''For the first couple of years,'' the author said, ''writing was a catharsis, and then I thought I really liked to write and could possibly make a living out of it. ... It turned from simply trying to tell myself and straightening out myself to communicating.''

Heinemann returned to college after the war and learned writing from the ground up. It took him six years to write ''Close Quarters.'' To support himself and his wife, he drove a city bus in Chicago, an occupation his father had had.

''It was the worse job I ever had, outside of Vietnam,'' he said. ''I was your bus driver nightmare come true. But I learned a lot about my father and why his disposition was so strange.''