Call of Duty 3 (Xbox 360 version)

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At the Chapter Select Screen, hold Back then d-pad right, right, left, left, then x, x. The effect only remains until you restart your console.

Achievements:

A War Hero (30): Capture the final objective in the game mode War. Must be a ranked match. (Multiplayer)
Allergic to Bullets (25): Avoid getting hit more than 30 times during a mission.
American Infantryman (20): Complete two missions as an American soldier.
Assault Trooper (15): Complete a mission by firing only assault rifles. Melee attacks don't count.
Basic Training (5): Complete the basic training in the beginning of Saint Lo.
Battlefield Scavenger (15): Complete a mission by firing only German weapons. Melee attacks don't count.
Big Air (25): Find a big jump and catch some air.
British Commando (20): Complete two missions as a British soldier.
Canadian Highlander (10): Complete two missions as a Canadian soldier.
Captain (40): Receive 2000 total points in ranked matches (Multiplayer).
Close Quarter Combatant (100): Complete a mission without firing a round. Player can make use of melee attacks and hand grenades.
Colonel (80): Receive 20000 total points in ranked matches (Multiplayer).
Conservationist (20): Complete a mission using less than 300 rounds of ammunition.
Doc (30): Revive 10 of your teammates in a ranked match without getting a teamkill (Multiplayer).
General (120): Receive 40000 total points in ranked matches (Multiplayer).
Grizzled Veteran (150): Complete the Single Player Campaign on Veteran difficulty.
Hot Potato (25): Pick up and return 5 live grenades.
Lieutenant (20): Receive 200 total points in ranked matches (Multiplayer).
Major (60): Receive 8,000 total points in ranked matches (Multiplayer).
Polish Tanker (15): Complete two missions as a Polish soldier.
Purple Heart (5): For perserverence despite grevious injuries.
Rifleman (15): Complete a mission by firing only bolt action rifles. Melee attacks don't count.
Still Ticking (30): Complete a mission without dying or using checkpoints.
Supply Officer (15): Supply ammunition to at least 20 friendly soldiers in a single ranked match (Multiplayer).
Victory Medal (30): Be the player with the highest score and on the winning team in a ranked match (Multiplayer).
Won the War (80):  Complete the single player campaign on any difficulty setting.

For more wartime fun, including a scope on all guns, head here.