The requirements for this glitch are: 2 people, death card Berserker, and the co-op mission Relentless (or any other one, but Relentless is the easiest).
First you start a private co-op game with death card Berserker on, start the game, run out there with your Browning and get Berserker (3 kills in 2 seconds).
Once both players are in Berserker the host quits, swiches to a zombies match (any map) aim down your sights (LT) and see what it does.

[unconfirmed]