The Heckler & Koch GMG ("GranatMaschinenWaffe" or "GraMaWa") is a 40mm automatic grenade launcher similar in form and function to the American Mk 19 series. It fires a high-velocity 40x53mm grenade in various warhead types to suit mission requirements. The high-velocity form contrasts the 40x46mm low-velocity series found in several shoulder-fired weapons. The high-velocity version is primarily reserved for vehicle-mounted guns. As such, the GMG is an oversized automatic grenade projector usually fitted to a heavy duty pintle mounting or tripod assembly.
Overall dimensions include a running length of 1,090mm with a barrel of 415mm long. The field version of the weapon consists of the 29kg gun system and 11kg tripod. The automatic nature of the weapon utilizes a recoil-based API (Advanced Primer Ignition) blowback action consistent with cannon caliber types 20mm and larger. Through the API action, the primer is ignited while the bolt is still in motion (moving forward) during which the cartridge is still being seated into the chamber. The bolt moves both forward and backward in the action, allowing for a reduced recoil force to be achieved which serves an automatic weapon such as this very well. The GMG can achieve a rate-of-fire of 350 rounds per minute and fires each grenade at a muzzle velocity of 790 feet per second out to an effective range of 1,500 meters and maximum engagement range of 2,200 meters. The GMG is fed by a 32-round disintegrating link belt usually held in an accompanying ammunition box. Feeding is through the right side of the receiver with spent casings ejected through a port on the left side. A carrying handle is fitted at the base of the barrel while additional aiming handles are present at the rear of the receiver. The barrel is capped by a long slotted muzzle brake. While fitted with standard iron sights, a reflex optical sight is also provided.
The GMG was designed by engineers at Heckler & Koch from 1992 to 1995 before seeing adoption by German Army forces in 1996. The series continues production today (2013) and is in the inventories of Canada (as the licensed-produced Rheinmetall-Canada "C16 CASW"), Finland (as the "40 KRKK 2005"), Greece, Ireland, Latvia, Lithuania, Netherlands, New Zealand, Norway, Poland, Portugal, Slovenia, the United Kingdom (as the "L134A1") and the United States (under SOCOM).