The commander of the Quantico Marine Base overruled an officer yesterday and promoted a 21-year-old military policeman seriously wounded by snipers last spring, even though the Marine lacked the necessary promotion points.

Cpl. James C. Jaeger, who is being forced out of the corps because of injuries he sustained in the shooting, will receive another honor today when he receives a Good Conduct Medal that Navy Secretary John Lehman specially authorized.

Even so, Jaeger, a native of Peoria, Ill., will be discharged Monday because he lost a lung as a result of the shooting and is considered unfit for further service.

"I feel bad about leaving because I wanted to make this my career," said the young Marine. His monthly disability pay will be $490.

Jaeger's case attracted attention after he complained he was "humiliated" when his battalion commander told him his disability rendered him ineligible for promotion, even though he was wounded while on duty. Quantico's spokesman, Maj. James Pendergast, said the officer was mistakenly applying a medical regulation that applies only to officers.

Pendergast said that Jaeger, an enlisted man, indeed had not accumulated sufficient points for a normal promotion. But when Quantico commander Lt. Gen. Richard E. Carey "heard about this he decided to promote him anyway because there was no doubt in the general's mind that he would have accumulated them if he was physically qualified," said Pendergast.

Jaeger's assailants were recently convicted of shooting into an occupied building and sentenced to 10 years in prison.