If developers put something inside of a game, players will find it. Even if it requires pulling the game apart, or in this case, going out of the bounds of a level.
AH Community recently detailed how to find a special gun that exists nowhere else in Advanced Warfare. Turns out, if you go to a level called Throttle, and you go into a specific building that's way off the path you're supposed to go in, you'll find this:
It's an RPG that is curiously named WEAPON_RPG — which makes me think the gun isn't supposed to be there in the first place. But, you can use it!
AH Community notes that the gun is somewhat glitchy, and that it is completely silent when you shoot it. Kind of funny, considering its the least technologically advanced gun in the game.
If you want to know how to get the gun yourself, watch this — but skip to about the :23 second mark, so you don't hear a random scream at the start of the video. You've been warned, headphone users!