crimestory

Actually R. Lee Ermey was a real Marine drill sergeant, and i believe his first on screen role was full metal jacket.

That's what I meant. Because he was a Marine drill sergeant, he was hired as a technical adviser for the movie to help the actor they had originally hired to play the drill instructor in the movie. The producers quickly realized that there was no way their actor was going to be better than R. Lee Ermey already was, so they fired the actor and hired Ermey to play the roll.