GP-25

The GP-25 is an underslung 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.

Multiplayer
This weapon is an attachment for the AK-47 assault rifle, both of which are available from the start, although rank 4 is required to make a custom class that uses the weapon. This attachment replaces the tier one Perk. 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.

Call of Duty: Modern Warfare 2
The GP-25 returns, in Modern Warfare 2. It is similar to the M203 in damage and effect. Unlike the M203, the SAS's and Marines' grenade launcher, in which the grenade compartment slides forward to accept/release a grenade shell, the GP-25 simply has a hole in front, which the user snaps his wrist to release a spent grenade, and just drops in a fresh one. It is called "Mortar-like" loading.

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).

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 Semper Fi loading screen.
 * The AK-47 is the only gun in Call of Duty 4 and Call of Duty: Modern Warfare 2 to use this grenade launcher. 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.