Turns out, when Ermey was a teenager, he got arrested twice for criminal mischief. The judge told him that he could either go to jail or join the Marine Corps. After serving for 11 years, he became a technical advisor for the movie industry. He took on this position with the goal to show filmmakers that he should be starring in their movies. As you can see, his plan worked out quite perfectly. Ermey persuaded Kubrick to cast him in Full Metal Jacket by making a homemade audition tape capturing him screaming insults.
