3D games are usually fraught with "secret rooms" that the developers never planned on gamers being able to access. Silly developers. This video will teach you how to glitch into a secret room in the War Machine map in Gears of War 2. You can't really see shoot at people from this one, but it's still cool to get there and know that you've been everywhere in the game.
You'll need to use this glitch to make this work.