GP-25

The GP-25 is an under-barrel grenade launcher seen mounted on the AK-47 in Call of Duty 4: Modern Warfare and Call of Duty: Modern Warfare 2.

Singleplayer
It is an attachment featured on the AK-47 Grenadier used by the Loyalists, OpFor and the Ultranationalists during the Singleplayer Campaign. Sergeant Kamarov is featured using this weapon. There is an AK-74u in "FNG" that can be seen with a GP-25.

Multiplayer
This is an attachment for the AK-47 assault rifle, which is available from the start, although rank 4 is required to make a custom class that uses the weapon. As previously mentioned, it is indistinguishable from the M203 attachment in function and damage. It does have a slightly longer reload time though. The majority of servers and players look down upon the use of grenade launchers as they are extremely effective and require minimal finesse to attain kills. It has earned the nickname "noob tube" and is referred to as such almost exclusively, even by players who use the attachment. Like the M203, it can be used to attain the challenge "Ouch" by directly hitting and killing an enemy player within the minimum range, which does not trigger the explosive fragmentation, but simply impacts the target, causing an instant death. The player can switch to the GP-25 slightly faster than the M203.

Singleplayer
The GP-25 returns in Modernsgsgsgsgsr

Multiplayer
The GP-25 is only available for the AK-47. It has slightly longer reload but slightly faster swap-to when compared to the M203 grenade launcher, but is otherwise the same. When equipped, it gives the AK-47 leaf sights, although they are unusable.

Call of Duty: Black Ops
The GP-25 returns as the AK-47's and Galil's grenade launcher. It is commonly available throughout the campaign.

Trivia

 * During the Call of Duty: Modern Warfare 2 reveal trailers the M203 was used as a place holder for the GP-25. This can also be seen on the Sniper Fi loading screen.
 * Oddly enough, in Modern Warfare 2, if you are equipped with the AK-47 Grenadier, you can see it slightly tilts right.
 * The AK-47 is the only gun in Call of Duty 4 and Call of Duty: Modern Warfare 2 that is usable when equipped with this attachment. It is not equal stat-wise to the M203 in either game, it has a faster switch-to time but a subtly slower reload time. Both are more apparent in Call of Duty: Modern Warfare 2.
 * When the GP-25 is attached, a small part of the grenade launcher is visible to the right of the iron sights. This is the leaf sight for accurately aiming the grenade. Also, the max ammo capacity increases from 300 to 308 (COD4 confirmed).
 * In Call of Duty: Modern Warfare 2's Singleplayer the GP-25 is most useful against riot shield enemies when fired to the side of or behind them.
 * It is interesting to note that when the AK-47 has this attachment, it is called the "AK-47 Grenadier." All other assault rifles, which use M203s, are denoted as "(weapon name) Grenade Launcher" (except the M4A1 and ACR).
 * The GP-25 uses a Grip in both Call of Duty 4: Modern Warfare and Modern Warfare 2, but it does not reduce recoil.
 * Black Ops DS is the only game with both the AK-47 and grenade launcher attachments that doesn't use the GP-25 for the AK-47.
 * The Galil is the first weapon besides the AK-47 to use the GP-25 grenade launcher.