AT4

The AT4 is a shoulder fired, disposable anti-tank rocket launcher featured in Call of Duty 4: Modern Warfare and Call of Duty: Modern Warfare 2.

Singleplayer
The AT4 is seen on allies, but is only usable by cheats. It's only used by allies twice; both times in "Shock and Awe". The first one is after some allies are let out near the statue. One of them has an AT4 and will fire it after hiding behind a wall. He's very close to the player, so they should see him get into position and fire before the player takes off. The last time is when the player gets to the stranded allies in the under-construction building; one of them will fire it into the crowd of troops right before the player gets to the top.

The AT4 can be used on the levels "Shock and Awe" and "Game Over" by cheating (this will only work on the PC version). The weapon, which is normally dropped by NPC's after firing, has no reloading animation. So when it is fired by using the cheat, it is just suddenly reloaded and ready to fire again.

Multiplayer
In multiplayer, the AT4 was originally supposed to be given to the player as a first tier perk. The perk's icon still remains in the game files, along with a kill picture.

Call of Duty: Modern Warfare 2
The AT4 debuts as a usable weapon in Call of Duty: Modern Warfare 2. This version is called the AT4-HS (Heat-Seeking Launcher) as it is capable of locking on to aircraft. To lock-on, the weapon must be ADS aimed at a aircraft until a clear, loud tone is heard and then fired.

Singleplayer
The AT4 can be seen and used during "The Only Easy Day... Was Yesterday" and in "Just Like Old Times". It is referred to in the pickup prompt as simply "AT4," and is only capable of free-fire in the Campaign mode. In "The Only Easy Day... Was Yesterday", the AT4 can be found before the player breaches the second room, and also inside the second breach point itself. A third AT4 can be found in the hallway near where the helicopter attacks the team. In "Just Like Old Times," the AT4 can be found in a few places such as outside the explosive-rigged control room.

Multiplayer
This Launcher is the first to be unlocked, at level four, and is also part of one of the default classes. Unlike the single player mode, the player does not discard the aparatus after firing the rocket. It is capable of both free-fire and lock-on, although it doesn't do as much damage as the Stinger or Javelin to any aircraft. It does the same amount of damage as the RPG-7 against infantry, although with a smaller blast radius and only one rocket. However, it is far more accurate, especially at range, and can be lock-on fired. Like the others, however, the player must be aiming down the sights in order to fire.

The AT4, alongside the RPG-7, does a very specific amount of damage against Harriers; the aircraft's max health (3000) minus 900.

Trivia

 * In Modern Warfare 2, the rocket's velocity is 6.31 m/s, or 21 feet per second.
 * This is the only launcher in-game capable of both lock-on and free-fire.
 * Like the Javelin, the AT4-HS has slightly unusual reloading behavior.
 * In Modern Warfare 2's singleplayer, the player tosses the whole apparatus away after firing; in multiplayer, however, the weapon is not thrown away, and can be reloaded.
 * The shrill lock-on sound heard when targeting aircraft can be heard when the AT4 is aimed toward an enemy.
 * When the weapon is fired in the first campaign, it won't explode when it reaches its full range, unlike the RPG-7.
 * At close range it is possible to get a headshot with the AT4

Video
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Video showing the accuracy of the AT4; it is accurate enough to get a headshot at medium ranges