How do you unlock a crossbow weapon on Assassin's Creed?

In Battlefield 3's online multiplayer mode, the player can unlock various weapons, gadgets and other items by ranking up or by completing assignments.

The Crossbow, or XBOW, is unlocked by completing the Sticks and Stones assignment. The assignment requires the player to win three rounds in the new Scavenger mode. The Crossbow can only be unlocked in the Aftermath expansion for Battlefield 3.