Friendly Fire

Team Killing (or "TK" for short) is a term used for a player accidentally or intentionally killing a team-mate. Team killing has been popular among many and dates back to the first Call of Duty. Today, Team killers are often found in online Hardcore matches, where friendly fire is enabled.

Team killing is often done accidentally, which in turn can lead to it being used as a form of revenge to reciprocate a team kill, or for any number of reasons including "griefing" (to deliberately harass other players).

Other players tend to kill teammates that are in Last Stand/Second Chance, so that the enemy won't get points for the kill.

Call of Duty & Call of Duty 2
Servers on the original online multiplayer and its sequel could or couldn't have friendly fire turned on. At the time there were four options: "Disabled" in which the player could not kill teammates; "Enabled" which meant the player could kill teammates; "Reflect" which is popular since team-killers will actually kill themselves rather than teammates; and "Shared" which is a combination of Enabled and Reflect.

Call of Duty 3
In Call of Duty 3, friendly-fire can now be responded to; if a person is team-killed they will now get the option to either punish or forgive the player. Punishing takes away 3 points from a player's score, often resulting in negative scores on the scoring boards, such as -15 for five team kills and no legitimate kills. If forgiven, the team-kill will have no effect, as it might have been by accident; however, it is up to the killed player to forgive or punish, so an accidental kill may result in a punishment anyway. Players are kicked after 9 team kills.

Call of Duty 4: Modern Warfare
The Call of Duty 4: Modern Warfare manages team kills more simplistically. And the normal Team Deathmatch has no Friendly Fire enabled, unless done so on a Private Match. The only game modes where friendly fire is possible are "Hardcore Team Deathmatch", "Hardcore Headquarters" and "Hardcore Search and Destroy". In Hardcore Team Deathmatch, the player has no choice over whether or not they are punished or forgiven. Often, Team Killers will enter Hardcore Search and Destroy with RPG-7 x2, Sonic Boom, Martyrdom. At the start of a match, they take out the RPG-7 and blast the ground, killing everyone around them. Hardcore Search and Destroy is one of the most popular Team-killer modes in the game only for the reason it has Friendly Fire enabled and that the player doesn't respawn. Upon two team kills the player will automatically die and every other team kill thereafter.

Call of Duty: World at War
Many aspects of team-killing are the same. Hardcore mode is still the only one with friendly fire enabled. Team-killers will often enter Hardcore playlists armed with Bazookas, lots of Grenades, Satchel Charges, and any gun they carry and wreak havoc on teammates. Upon killing three teammates the player will be kicked unless they are the host; the host, however, is free to teamkill with no limits. If a player kills a teammate within a few seconds after a match starts, they will die as well and will lose 1 point for a suicide as well as the 10 points for team-killing.

Call of Duty: Modern Warfare 2
Team killing is still possible in Call of Duty: Modern Warfare 2 in the Hardcore game modes. However, there is a Hardcore Ricochet: S&D that prevents players from killing their teammates by reflecting the bullets and killing the player. In private matches, players will never get penalized for friendly fire. They are free to constantly team-kill with no limits. The most common team killing in this game is for Care Package and/or Emergency Airdrop crates, as players will wait for the package to come down and kill the owner if there is a high killstreak inside, such as an AC-130 or EMP. However, the new game mode, Hardcore Mosh Pit has multiple games without "Shared" on, so teamkillers are far more likely to join this instead of Hardcore Team Deathmatch.

Killing a teammate right at the start of a game will result in the player instantly dying as well.

Call of Duty: Black Ops
Call of Duty: Black Ops team killing remains same as Call of Duty: World at War, with team killers being kicked from the game. As in Call of Duty: World at War, the faction announcer will make a remark when someone gets kicked, for example, the Call of Duty: Black Ops announcer makes remarks like, "Son of a bitch isn't fit for the mission, get him out of here!" or "Get that son of a bitch out of my sight!"

Call of Duty: Modern Warfare 3
When Call of Duty: Modern Warfare 3 was released team killing was allowed in all Hardcore game modes, however due to players abusing the ability (especially in Search & Destroy) this was soon patched, causing damage to reflect onto the shooter (Ricochet). The game modes where also retitled to match (Hardcore Team Deathmatch became Hardcore Ricochet: TDM). The most common form of teamkilling is jumping in front of a firing teammate, whether it be a launcher or through bullets, resulting in the death of the shooter. The other most common form of teamkilling is when the player sets down any killstreaks or equipment a teammate can destroy it. As of October 5th, 2012, players can now teamkill again on the Hardcore Face Off 3v3 mode.

Call of Duty: Modern Warfare 3: Defiance
Team Killing makes its first appearance on the DS, and is a host-controlled function in Public Matches and Private Matches, unlike in console titles. The host has the option to have Friendly Fire turned on or off in both Public and Private Team-Based Matches. However, even if the host has Friendly Fire turned off, the player can still knife and throw knives at other teammates, possibly a glitch that wasn't meant to happen. Nevertheless, players cannot aim directly at a teammate when they use or throw their knife, but aim away from the teammate. If using a regular knife, when friendly fire is off, one must aim to the sides of the player, press the Fire button, then when the knifing animation is starting, aim at the teammate. When throwing knives at teammates when friendly fire is off, you must again aim away from the teammate, like when knifing, and when you just begin to raise the knife back to throw, aim at the teammate. Unlike its console counterpart, Defiance has no Ricochet, which they player may use to their advantage.

Call of Duty: Black Ops II
Call of Duty: Black Ops II featured a number of Hardcore game modes, and in private matches there is a server option for teamkilling, with the choices "On", "Off", "Reflect" and "Shared". There is a number of teamkills (default: three) one can commit before being kicked. When a player is kicked for this reason, the announcer may comment on the player.