TWENTYNINE PLAMS, Calif. - A Marine sustained a self-inflicted gunshot wound after a two-hour standoff with military police Tuesday on a sprawling Marine Corps base in the Southern California desert, military officials said.Shots were reported around 6:30 a.m., triggering an alert of a possible active shooter at Marine Corps Air Ground Combat Center Twentynine Palms, about 125 miles east of Los Angeles, said spokeswoman base Capt.