[svandamme]

Apparently he was the real deal :

Former U.S. Marine Drill Instructor Ermey was originally hired as a technical adviser and asked Kubrick if he could audition for the role of Hartman. However Kubrick, having seen his portrayal as Drill Instructor SSgt Loyce in The Boys in Company C, told him that he was not vicious enough to play the character. In response, Ermey made a videotape of himself improvising insulting dialogue towards a group of Royal Marines while people off-camera pelted him with oranges and tennis balls. Ermey, in spite of the distractions, rattled off an unbroken string of insults for 15 minutes, and he did not flinch, duck, or repeat himself while the projectiles rained on him.[6] Upon viewing the video, Kubrick gave Ermey the role, realizing that he "was a genius for this part".[8] Ermey's experience as a real-life DI during the Vietnam era proved invaluable, and he fostered such realism that in one instance, Ermey barked an order off-camera to Kubrick to stand up when he was spoken to, and Kubrick instinctively obeyed, standing at attention before realizing what had happened. Kubrick estimated that Ermey came up with 150 pages of insults, many of them improvised on the spot — a rarity for a Kubrick film. According to Kubrick's estimate, the former drill instructor wrote 50% of his own dialogue, especially the insults.[11]