When you start the Horde I recommend you get to the level 1-4. Then go where the carbine wall is and the boxes.

Let your partner jump between the boxes and the wall, then crouch and try to get beneath him.

If you do it right you should be combined with your partner. Now if don't move and knife or else you get out of the glitch.

If you are out of ammo you will see the Carbine right in your left.

Note: this just works on Natch der Untoten.