At first it was just another job. Now Jon Bullock can't get enough Iraq. Bullock, 38, of Fullerton, has been nearly blown up by mortars. He's been knocked on his butt by helicopter backwash. He's commuted to work in Fallujah sitting beside men pointing .50-caliber machine guns out open doors. And he's just a rock 'n' roll drummer. Bullock is one of dozens of Orange County musicians who have become nearly obsessed about playing for U.S. troops in Iraq. It's the camaraderie, they say. The opportunity to witness history. The feeling they get when a service member thanks them for...