Officer James Gustafson Jr., 23, had a blood alcohol level of 0.09 percent - slightly above the 0.08 limit for legally driving a car - when he shot himself Aug. 11, San Mateo County Coroner Robert Foucrault said today.

Gustafson shot himself in the neck at 1:40 a.m. during a gathering of as many as 15 people, as he was showing a woman how police are taught to prevent someone from using a gun against them, authorities said.

There's no way to know whether Gustafson was too drunk to know what he was doing, the coroner said.

"Somebody's blood alcohol level and their ability to function correctly can vary from individual to individual," Foucrault said. "It's subjective."

According to those familiar with the incident, Gustafson was showing the woman his Police Department-issued semiautomatic pistol after removing the magazine that stores the rounds. However, there was still a round in the chamber.

Gustafson pointed the weapon at his neck and pulled the trigger, shooting himself, according to authorities.

The coroner's office has ruled the case an accident. Police initially characterized the case as an accident but are awaiting official receipt of the toxicology results and gunshot residue tests before making a final determination, Lt. Mike Brunicardi said.

Gustafson graduated from the San Francisco police academy in January. He had recently finished a six-month stint in the Mission District under a field training officer and had been transferred to the department's Central Station.

E-mail John Coté at jcote@sfchronicle.com.