Call of Duty. Or, CoD. It's an abbreviation so ubiquitous it's kind of weird that nobody thought to do this sooner.
Indie dev Ryan Evans has made Cod of Duty, a short, simple game where you... shoot fish. You know. Cod. They're even (mostly) in a barrel, just to complete the whole Call of Duty experience.
Download it below on PC, Mac and Linux.
CoD of Duty: The Premier First Person Shooting Fish in a Barrel Simulator [Ryan Makes Stuff, via Rock, Paper, Shotgun]