A Smithsonian Institution security guard shot and killed himself with his service revolver last night in a basement security office at the Renwick Gallery, D.C. police reported.

Police identified the guard as Gary M. Proctor, 26, and said he was pronounced dead at the scene of a single.38 caliber bullet wound to the head.

The incident occurred about 5:30 p.m. after Proctor had come to the office to turn in his revolver and ammunition to a supervisor prior to checking off duty, police said.

Police said the supervisor noting that Proctor had given him only five of the six cartridges issued, asked about the other cartridge. The guard said he still had it in the pistol and intended to "go outside and shoot it into the air," but then continued, "on second thought I think I'll play a little Russian roulette," police reported.

Police said, the guard then placed the revolver to his head and pulled the trigger.

Homicide detectives investigating the incident said early today that the shooting may have been accidental or inadvertent. A final determination will be made by the D.C. medical examiner.

Police said Proctor, who lived at 8415 Hamlin St. Lanham, had been employed as a Smithsonian guard for at least two years. They said he was married and the father of a boy, 5, and a girl, 2.