M203

The M203 is a single shot 40mm grenade launcher that can be attached to a variety of weapons. The M203 was created during the Vietnam war to replace the single-shot M79 grenade launcher, allowing grenadiers to be armed with a potent backup weapon (The M16 it is mounted on) rather than have to switch weapons. It is currently in use with the US military, though it is in the process of being replaced in US Army service with the Heckler & Koch M320 Grenade Launcher Module (GLM). The launcher comes in a number of barrel lengths, such as the original 12' model for the M16, the 9' model for the M4 carbine, and a 6" M203 PI system that can be mounted on many weapons, including the MP5.

In-Game
In Call of Duty 4: Modern Warfare's multiplayer mode, all assault rifles except for the MP-44 and AK-47 can be outfitted with a M203 Grenade Launcher. The AK-47 uses the GP-25 Grenade Launcher, and the MP-44 cannot equip attachments. Interestingly, the G3, though compatible with the M203 PI variant, is only compatible with the normal M203 through modifications, and the G36C is not compatible with it at all. The M14, a weapon incompatible with the M203 in the basic form in which Call of Duty 4 presents it, would need incredible rail grafting to accommodate it. The M203 variant mounted on the M4 Carbine and the M16A4 also differ from each other, with the M203 mounted on the M4 being an M203A1/M203 QA, which is designed to mount on a RAS bracket. Meanwhile, the ones on the M16 are standard M203, which only differs than the M203A1 by barrel lengths; the A1 uses 9' barrel, while the standard M203 uses 12' barrel. The grenades it shoots are fairly weak with a small blast radius, but they are also extremely accurate, fast-moving, and have very little arc. Also, it does not need to be unlocked through challenges. For these reasons it is generally looked down upon as unfair by many players when used in multiplayer, and is often referred to as the "noob tube". Many servers on the PC version either ban this attachment, or ban those using it. This attachment replaces Perk 1 in multiplayer. Even though leaf sights are present on both the M4 and the M16 when the M203 is attached they are never used. Most likely this is to not unbalance the accuracy between M203s with iron sights and M203s without.

Trivia

 * If the Overkill perk is used, and both selected weapons have the M203 (or the GP-25) grenade launcher, then the player will have 4 40mm grenades (1 in each launcher and 2 spare ones for reloads).
 * The grenades launched by the grenade launcher are only armed after traveling a certain distance. If they strike a surface before traveling this distance, then they will bounce off rather than explode which keeps players from "noob tubing" at close range unlike Rifle Grenades in Call of Duty: World at War. It is possible to get the "Ouch" challenge with the M203 by hitting and killing an enemy this way.
 * Since the M203 is a weapon attachment, players cannot pick up more ammo for it by walking over a dropped gun with a grenade launcher attached.
 * If the player has a M203 (or a GP-25 in the case of the AK-47) mounted on any weapon, when the Unlimited Ammo cheat is used, the M203 becomes fully automatic. Interestingly, the ROF on an M16A4 is much higher than that of other rifles.
 * The M203 being in service in 2011 and 2016 is inaccurate, as it is slated to be replaced by the M320 Grenade Launcher Module, an underslung grenade launcher of similar design to the Heckler and Koch AG36.
 * It could be that the militaries were looking to use a cheap surplus weapon instead of the M320GL because there is a GLOBAL WAR going on with RUSSIA and USA.
 * The M203 has a slightly faster reload time than the GP-25.
 * In Call of Duty: Modern Warfare 2, after getting 20 kills with an M203 that is attached to a specific weapon, the Masterkey Shotgun will be unlocked for that weapon.
 * In the Multiplayer for Call of Duty: Modern Warfare 2, players can now use the grenade launcher from a picked-up weapon, unlike Call of Duty 4: Modern Warfare.
 * Strangely, in single-player mode, if fired at extremely close range, the grenade will pass through an enemy without detonating and kill the enemy.