GP-25

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

Modern Warfare 2
A grenade launcher used in both Modern Warfare games is GP-30, as evidenced by sights placed on the right side of the barrel. The GP-30 is similar to the M203 in damage and effect although in real life they function quite differently. The main difference is in loading mechanics and design of rounds that it fires. Apart from standard VOG-25 grenade, VOG-25P was developed, that features a small, initial charge which is triggered upon impact. This initial charge launches the grenade approximately 0.5 - 1.5 meters into the air before the primary fragmentation charge is triggered by the delay timer. This is quite different from the M203's 40mm grenade, which simply explodes upon impact, assuming it has been airborne long enough to arm the grenade. 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-30 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. In reality, the GP-series launchers use caseless grenades, which do not leave a shell in the barrel. This allows the shooter to simply load a new grenade without spending time to extract a spent case.

Trivia

 * During the Modern Warfare 2 reveal trailers the M203 was used as a place holder for the GP-25.
 * The AK-47 is the only gun in Call of Duty 4 and Modern Warfare 2 to use this grenade launcher. It is not equal stat-wise to the M203 in both games: it has a faster switch-to time but a subtly slower reload time. Both are more apparent in Modern Warfare 2.