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. Unlike it's 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". 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.

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. Thus it is impossible 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.

Also, when picking up a weapon with this attachment, any unspent grenades will be added to your inventory.

Getting 20 kills with an M203 that attached to a specific assault rifle, the Masterkey Shotgun will be unlocked for that weapon.

A little known fact is that it is a possibility to get a headshot with this attachment. Along with the Thumper, if you shoot the grenade and it explodes on contact with the head it will show the headshot indicator. The reason no one really knows is because there are only 1 - 2 videos where this happens.

If the perk Danger Close is used with the M203 is becomes very powerful and even more effective.

Trivia

 * It is possible to get the "Ouch" challenge with the M203 by directly hitting and killing an enemy at close range without having the shell explode.
 * 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 being phased out by the M320 Grenade Launcher Module, an underslung grenade launcher of similar design to the Heckler and Koch AG36 and FN40GL.
 * The M203 has a slightly faster reload time than the GP-25.
 * In Modern Warfare 2, it is possible to get registered headshots with the grenade launchers (M203/GP-25)
 * In real life this weapon has been used on the AK-47 but it was hard to use properly.