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 having 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. Unlike its Russian equivalent the GP-25, the M203 is breech-loaded.

In-Game
In Call of Duty 4: Modern Warfare's multiplayer mode, all assault rifles except for the MP44 and AK-47 can be outfitted with a M203 Grenade Launcher. The AK-47 uses the GP-25 Grenade Launcher, and the MP44 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, and even then, the bulky form of the weapon would not accommodate it well. 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 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 will not detonate unless it has traveled a certain 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). 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.

Modern Warfare 2
This attachment is mostly the same except now it does not take up a perk 1 slot. This means it can be used with perks like Bling, 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, however the player must switch to the launcher to pick up more ammo for it even if it's empty.

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

 * A little known fact is that it is possible to get a head shot with this attachment. Along with the other launchers, if the user shoots an explosive and it explodes on contact with the head it will show the headshot indicator.
 * With enough practice, it is possible to completely protect flags from across the map during a game of domination through effective use of drop shots.
 * It is possible to get the "Ouch" challenge with the M203 or GP-25 by directly hitting and killing an enemy at close range without having the shell explode. In Modern Warfare 2, the challenge comes with the rather humorous title, "Noob Tuber." This title suggests that Infinity Ward has recognized the use of the "Noob Tube" as frowned upon amongst the community and considers it funny in a "cute" sort of way.
 * Oddly, if the player use the FAL with a grenade launcher, it is spelled "FAL Grenade Laucher" instead of "Launcher". This appears to be an oversight by Infinity Ward.
 * 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 in Campaign, the M203 becomes fully automatic. Interestingly, the ROF on an M16A4 is much higher than that of other rifles.
 * The M203 has a slightly faster reload time than its AK-47 counterpart, the GP-25.
 * 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 Modern Warfare 2, the M203 has no visible trigger guard while it is equipped (Easily seen while reloading the FAL).
 * In campaign when the M203 is attached to weapons like the M4A1 and the SCAR-H it can hold a total of 11 40mm grenades, but when attached to the M16A4 the player can only hold a total of 4 grenades. This is probably a glitch missed by Infinity Ward.
 * If you have a FAL with either a M203 or a Masterkey Shotgun, if you reload, it looks like the empty magazine flies through the Shotgun or M203.
 * 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 some weapons, if a Heartbeat Sensor is used on a weapon with the Grenade Launcher, the Heartbeat Sensor covers the Grenade Launcher cross hair. The FAL is an example of this.
 * The Grenade Launcher can bounce twice off of surfaces and still kill on regular gametypes.