What the heck was he using, a pellet gun?
No, it was a regular handgun. You can read about the case in this book. I gave away my copy to someone who is studying forensics in college, so I can't check if the caliber was mentioned. The book said that the man's eyes were still moving and following the police as they entered the room. His secretary was pounding on the locked door as she heard the five shots. I figure that his temporary survival depended on the specific paths that the bullets took through his head.