In Call of Duty 4: Modern Warfare, the AN/PEQ-6 is used exclusively during the single-player campaign. It is mounted on all M4A1s, regardless of whether the player has night vision goggles.
It may be noted as a flaw on Infinity Ward's part that the beam itself is emitted from the muzzle of the weapon on the M4A1 Grenadier and the M4A1 SOPMOD, instead of one of the device's lenses.
The AN/PEQ-6 laser mounted on the M4A1 Grenadier
The AN/PEQ-6 laser mounted on the M4A1 SOPMOD
The AN/PEQ-6 laser beam, seen through night vision goggles
Call of Duty: Modern Warfare 2
In Call of Duty: Modern Warfare 2, the AN/PEQ-6 is used in "Exodus" to mark targets for the Stryker. It is also seen in "The Gulag" and "Just Like Old Times" when Night Vision Goggles are used. It is also attached to all weapons available to the player on the ground in "Overwatch". It is also seen being used by Shadow Company. The AN/PEQ-6 can be seen above the Intervention's barrel, but it is removed when equipped with an ACOG Scope.
The AN/PEQ-6 can be found in the mission "Stronghold" equipped on an MP5. It is also present on various weapons in Special Ops.
Also, when enabling Console commands (only on PC), it is possible to activate laser both in Singleplayer and Multiplayer. On Akimbo weapons, there will be two laser beams. It also appears in the special ops mission "Iron Clad" on solo, to mark targets for the tank.
Call of Duty: Ghosts
The AN/PEQ-6 is seen in the level "Clockwork", where many of the Ghosts have it attached to their weapons. The player, however, does not.
Trivia
Call of Duty: Modern Warfare 2
The laser beam is visible without the use of night-vision goggles.
Call of Duty: Modern Warfare 3
In the Stronghold, while equipped on Yuri's MP5, the AN/PEQ-6 appears much smaller than in previous games.