M203

The M203 is a single shot grenade launcher attachment that is featured in Call of Duty 4: Modern Warfare and Call of Duty: Modern Warfare 2.

Call of Duty 4: Modern Warfare
In Call of Duty 4: Modern Warfare's multiplayer mode, all assault rifles except for the MP44 and AK-47 can be outfitted with an M203. The AK-47 uses the GP-25 Grenade Launcher, and the MP44 cannot equip attachments. The grenades it shoots are slightly weaker than frag grenades, 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".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.

The M203 shell has a safe arming distance of 5-6 Meters and will not detonate unless it has traveled this distance before hitting a surface. Thus, shooting at nearby walls, ground or ceiling will simply cause the shell to bounce harmlessly away. This makes using the M203 at close ranges very difficult. However, the shell will kill if it directly hits another player and has a special kill icon for kills achieved this way.

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), except in the Wii version, which sets it at only 3. 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 (unless they currently have the empty attachment up while doing so, prompting a reload). It can be difficult to gain more ammo for the M203 without respawning.

Call of Duty: Modern Warfare 2
This attachment is mostly the same except now it does not take up a Tier 1 slot, unlike Call of Duty 4. Sleight of Hand affects the reload speed effectively increasing its rate of fire. With the introduction of the Scavenger or One Man Army perk, one can technically acquire infinite numbers of grenades. It is notable that if the player is at maximum ammo with the rest of their weapons and grenades (common when using Scavenger Pro), they must have the grenade launcher equipped to pickup more ammo with Scavenger. This can make picking up replacement grenades in combat troublesome if the player uses their grenade launcher first or exclusively.

Using this attachment is still considered "Noob tubing". In fact, there is a title that has the text "Noob tuber".

Many people hate this attachment (especially when used with "Danger Close"), because they believe it is overpowered, and noobish. They call the people who use it "Noob tubers".

Getting 20 kills with an M203 attached to a specific assault rifle unlocks the Masterkey Shotgun for that weapon. Also, when picking up a weapon with this attachment, any unspent grenades will be added to the users inventory.

One frequently used, but heavily frowned upon tactic with the Grenade Launcher is to use the One Man Army perk with an Assault Rifle with a Grenade Launcher attachment and switch to the same class to gain infinite grenades and combine that with the Danger Close Perk. Some players however use the Scavenger perk because it allows a Rocket Launcher to be equipped, and because of the recent update to the One Man Army class which increases the time it takes to change classes if used more than once per life.

Trivia

 * In the console multiplayer mode of Modern Warfare 2, the M203 is accompanied by a vibration in the controllers when fired. However, in Single Player, only the M4A1 and the M16A4 has the vibration effect while other grenade launcher-capable rifles do not.
 * In campaign when the M203 is attached to weapons like the M4A1 and the SCAR-H it can hold a total of 11 grenades, but when attached to the M16A4 the player can only hold a total of 4 grenades. This is more than likely an oversight.
 * When firing either of the two underbarrel grenade launchers, M203 or GP-25, the muzzle blast comes from the rifle barrel, not the grenade launcher's barrel.
 * On the Wii version of Call of Duty 4, the reticule for the grenade launcher is different, looking similar to the Holographic Sight.
 * It is possible to get a headshot with an M203 Grenade Launcher