The GP-25 Kostyor (meaning "Bonfire") is a Russian 40mm underbarrel grenade launcher that is commonly fitted onto weapons of the AK series.
In 1966, development of a grenade launcher for the AK assault rifle was started at the Sporting and Hunting Arms Central Design and Research. Development of the weapon continued into the 1970s, and it was accepted into military service in 1978 for use on the AK-74 and AKM assault rifles.
It was replaced by the GP-30 grenade launcher in 1989.
The GP-25 uses the High-Low System, developed in WWII by Germany, to keep recoil forces low without any back blast caused by rocket-based or recoilless weapons.
It also features a notched quadrant sight with an indirect firescale on the left-hand side of it that is adjustable from 100 to 400 meters.
The barrel of the weapon is rifled. Grenades are loaded from the muzzle and held in the barrel by a spring catch. Unfired grenades are extracted using an integral push-out extractor, located at the left side of the weapon. The firing mechanism is of double-action type, with a manual safety and several automated safeties, which prevents firing if the launcher is not properly installed on the host rifle or if the grenade not completely loaded.
The GP-25 fires a series of special 40×53mm grenades. The grenade has both the projectile and casing consolidated into a singular solid piece, in a stark contrast to the two-piece casing-and-projectile system used by the American 40×46mm rounds used in breech-loaded launchers such as the M203.
Because of this layout, it is considered 'caseless', but not in the same context as ammunition used by weapons such as the Heckler & Koch G11 assault rifle, but in the context where no spent casings are left behind after firing. This made reloading much less of a chore. A similar layout is seen in the 9×25mm AUPO ammunition used by the Benelli CB-M2 submachine gun.
- VOG-15 (7P17) - The original main grenade, a fragmentation round.
- VOG-25 - Fragmentation grenade and successor to the VOG-15. It has a lethal blast radius of 6 meters.
- VOG-25P - A bouncing variation, which has a nose-mounted charge that raises 0.5 to 1.5m in the air before detonation by impact delay fuse. Its lethal blast radius is also 6 meters.
- GP stands for Granatomyot Podstvolnyj, which means "underbarrel grenade launcher", befitting of the weapon's role.