Call of Juarez: Bound in Blood

Keep on playing and taking down bad guys with the rifles that have "x2" in their name/description. Works for both singleplayer and multiplayer modes. Remember you can have a look at your progression at any time by checking out the "achievements" section in the main menu.

I suggest using the Trapper in online play. His shotgun has decent range, and you can score some easy kills on most of the maps setting at the tops of stairs. This is helping me out right now.