FOR PEOPLE WHO HANDLE FIREARMS, whether for work or recreation, there is one principle that may never be ignored: Treat every gun as if it were loaded. Robert Plante was just 29 when he broke that rule on Nov. 10. He paid with his life, one of more than 12,000 people who are killed by firearms in the United States every year.

Yet, of all those who accidentally die by the gun, Plante was one of the least likely candidates. He was an expert marksman who had been around guns since childhood. He was vying for a place on the U.S. Olympic shooting team that will compete in the Olympics in Barcelona next year. And he was strict about firearm safety.

Plante, a native of Westland, Mich., had worked as a machinist until January 1990. Then, to concentrate on his training, he moved into the U.S. Olympic training center in Colorado Springs, where he was provided with room and board. Plante was cheerful and popular, with a reputation as an incorrigible prankster. He was, however, serious about guns and gun safety, about which he lectured to Boy Scouts, Big Brothers/ Big Sisters of America and other groups touring the center. "He was very careful," says teammate Neal Caloia. "Firearm safety was second nature to him."

On Nov. 10, Plante stopped off at the home of Lou Kosich, an acquaintance who lives in nearby Parker, Colo., to pick up a pistol that he was buying from Kosich. With Plante were two members of the Spanish national shooting team. "One of the Spaniards was sitting next to Robert," says Dan luga, the U.S. team's pistol coach, "and the other was across the table from him. Kosich was farther down the table, filling out some papers. No one was looking at Robert. They said Robert picked up a Colt .45 automatic pistol he had brought with him, jokingly put it to his head and said something like 'Hasta la vista, baby' and pulled the trigger. They heard a shot, and Robert fell forward on the table. I guess he thought the gun wasn't loaded."

Unfortunately for Plante, a bullet offers no second chance, as he himself knew only too well. He is survived by his parents, Don and Jan, and by two sisters and a brother.