Well, assuming they got his rank correct in the story -
He's still in the Corps, he's a designated marksman, and obviously not a slacker, so I would guess he was once a Cpl or Sgt who did something like hit someone upside the head for one good reasonor another and got NJP and a bust in rank for it...
Yes, a few others have suggested as much.
(BTW, great screen name! LOL!)