Grenade Launcher

In Call of Duty 4: Modern Warfare, all assault rifles except the MP44 can have a Grenade Launcher attached, the M203 for most assault rifles and the GP-25 Kostyor for the AK-47. Call of Duty 3 and Call of Duty: World at War have the Rifle Grenade for the same purpose.

Call of Duty 4: Modern Warfare
The Grenade Launcher is unique in that it is the only attachment that is unlocked without having to complete any challenges. The GP-25 functions the same as the M203 with only cosmetic differences. Using a grenade launcher takes up the first perk slot. It is often referred to as the "noob tube", because it is capable of killing players easily, and does not need to be aimed directly at the player. Players can be killed either by the grenade's explosion or by being hit by the grenade directly. Enemies who are too far from the point of detonation will only be injured. The grenades are incredibly accurate and move quickly, but have a small blast radius. The grenade must travel a certain distance before it will explode, else it will only bounce off the surface it impacts. Players carrying a single grenade launcher will carry two grenades for it. Players carrying two weapons with a grenade launcher (using Overkill) will either have three for one and one for the other (if the two weapons use compatible ammo) or two for both.

The G3 and G36C still use the M203 even though the G36 is normally issued with the Heckler & Koch manufactured AG36 grenade launcher, and requires a modified rail mounted M203. The G3 also requires modification to fit an M203.

The Mk-19 granade launcher is use in the mission "Shock and Awe" on the transport helicopter and is used to to lighten the ground resistance and with a few shots can easily destroy an A.P.C.

Call of Duty: Modern Warfare 2
The grenade launcher was left pretty much the same in Modern Warfare 2, although using a launcher no longer uses a perk slot instead it uses an attachment slot (cannot be combined with the shotgun attachment). Also the grenade launcher requires completion of Marksman Rank 1 before it can be used (on most guns). Killing enemies with the launcher is also a requirement for unlocking the shotgun attachment.

M203
The M203 is a single shot 40mm grenade launcher that can be attached to a variety of weapons. It is currently in use with the US military, though it is in the process of being replaced with the Heckler & Koch M320 Grenade Launcher Module (GLM).

GP-25
The GP-25 is a Grenade Launcher that originated from the Soviet Union. It is used on the AK-47.

M79
The M79, aka "Thumper", is a stand-alone grenade launcher that appears as a secondary weapon in Call of Duty: Modern Warfare 2. It uses the same grenades as the M203, and comes with two rounds. The M79 was introduced in Vietnam as the primary weapon for grenadiers and was replaced by the M203 for the M16 rifle but remained in service in several countries.

Trivia

 * If the player shoots a grenade at a nearby wall or something that it will bounce off of, they will be able to see the unexploded grenade on the ground.
 * The Grenade Launcher (in all its forms, attachment or otherwise) is often called the Noob Tube by online players.
 * The correct grenade launcher for the G3 is the HK79.
 * The correct grenade launcher for the G36 is the AG36.
 * The correct grenade launcher for the F2000 and SCAR is the EGLM.
 * It has also been deemed the "noob tube" on multiplayer in MW and MW2 due to the easy kills on small maps.