- "Each player is given a pistol carrying just one bullet. Every landed shot kills. You earn points with each kill, as well as an additional bullet. Get killed three times and you’re out for the remainder of the match. The player with the most points at the end of the game wins."
- — Call of Duty ELITE description.
One in the Chamber is a Wager Match game mode featured in Call of Duty: Black Ops, part of FFA Gunplay in Call of Duty: Modern Warfare 3, and an optional match mode in Call of Duty: Modern Warfare 3: Defiance.
Call of Duty: Black Ops
One in the Chamber consists of six players competing in a Free-for-All style game with a twist: each player starts off with only an insta-kill M1911, a single round of ammunition, a knife, and three lives. Players are equipped with Lightweight, Scout (though it has no effect), and Marathon Pro.
Once a player has fired their bullet, the only means of acquiring another one is by getting a kill. This means that if a player kills an opponent with a bullet, they will get that bullet back. However, if a player happens to have no ammo left, then their only remaining method of killing is by knife. Knifing earns one bullet, which means that players can have more than one bullet at a time if they choose to knife their enemies as opposed to shooting them.
At the end of the match, a Spy Plane will appear to show both players on the radar as a way to prevent camping which would excessively prolong the match.
The winners of One in the Chamber are the players with the highest scores. Kills earn 100 points, while Survivor Bonuses (being alive when another player is eliminated) net 10 points. While it may seem as though it's a Last Man Standing setup, it's easily possible to be in the money without being one of the last few alive.
Call of Duty: Modern Warfare 3
One in the Chamber returns in Call of Duty: Modern Warfare 3 originally only as a Private Match-exclusive mode. Like in Black Ops, the player gains ammo by killing enemy players with a one-hit-kill (can be disabled) shot or with a knife, but here the weapon featured by default is the Desert Eagle, and the player starts with 2 lives (both of these values can be changed in the Private Match Mode options).
On March 30, 2012, One in the Chamber was added to the Community Playlist as part of FFA Gunplay. In this gametype, the P99 with Tactical Knife is used. Once the player dies three times, they can spectate the remaining players and choose one to back. When a spectator backs a player, the spectator gains 150 XP for each kill the player they're backing makes. When there are only three players remaining, a UAV will activate for all players.
Call of Duty: Modern Warfare 3: Defiance
One in the Chamber, for the first time in the Call of Duty DS titles, is added as an optional match mode. Similar to Call of Duty: Black Ops and Call of Duty: Modern Warfare 3, up to six players compete in a Free-for-All game. Each player is equipped with a Desert Eagle with only one bullet and their combat knife. (throwing knives have been disabled)
The rules do not differ much from the console version, but the match time limit as well as point limits can be set before match.
- In the original Wager Match trailer, One in the Chamber originally featured a Makarov, rather than an M1911. The One in the Chamber icon still has a Makarov however.
- The maximum amount of bullets a player can have in their pistol is seven, which is the same as M1911's ammunition capacity in multiplayer.