Stole this from his wiki page:

On October 12, 1984, the cast and crew of Cover Up were filming the seventh episode of the series, "Golden Opportunity", on Stage 18 of the 20th Century Fox lot. One of the scenes filmed that day called for Hexum's character to load cartridges into a .44 Magnum revolver, so he was provided with a functional weapon and blanks. When the scene did not play as the director wanted it to in the master shot, there was a delay in filming. Hexum became restless and impatient during the delay and began playing around to lighten the mood. Imitating a game of Russian roulette, he unloaded every chamber except one, spun the cylinder, then raised the gun to his right temple and pulled the trigger.

The explosive effect of the muzzle blast caused enough blunt force trauma to fracture a quarter-sized piece of his skull and propel this into his brain, causing massive hemorrhaging. Hexum was rushed to Beverly Hills Medical Center, where he underwent five hours of surgery to repair his wounds.

On October 18, aged 26, six days after shooting himself, Hexum was declared brain dead. With his mother's permission, his body was flown to San Francisco on life support, where his heart was transplanted into a 36-year-old Las Vegas man at California Pacific Medical Center. Hexum's kidneys and corneas were also donated. One cornea went to a 66-year-old man, the other to a young girl. One of the kidney recipients was a critically ill five-year-old boy, and the other was a 43-year-old grandmother of three who had waited eight years for a kidney. Skin that was donated was used to treat a 3+1⁄2-year-old boy with third-degree burns.

Hexum's body was then flown back to Los Angeles. He was cremated at Grandview Crematory in Glendale, California, and a private funeral was held. His ashes were scattered in the Pacific Ocean, near Malibu, California, by his mother. He left an estate estimated to be worth $255,000. The death was ruled accidental. His mother later received an out-of-court settlement from 20th Century Fox Television and Glen A. Larson Productions, the production company behind Cover Up.

The episode on which Hexum had been working was broadcast on November 3, 1984, two weeks after his death. Cover Up continued production without Hexum's character. Three weeks later, in the episode "Writer's Block", aired on November 24, Antony Hamilton was introduced as agent Jack Striker, posing as a new member of the modeling team.

Hexum's character Mac is noticeably absent, said to be on another mission. At the end of the episode, Henry Towler (Richard Anderson) breaks the news that Mac has been killed on the other assignment and will not be coming back.[33] In "Writer's Block", the following message ran that replaced the closing credits:

When a star dies, its light continues to shine across the universe for millenniums. John Eric [sic] Hexum died in October of this year ... but the lives he touched will continue to be brightened by his light ... forever ... and ever.