The Mk 19 grenade launcher (also known as the Mark 19) is an American 40 mm belt-fed automatic grenade launcher that was first developed during the Vietnam War. The first model (Mod 0) in 1966 was determined to be unreliable and unsafe, but a total of six Mod 1 launchers were successfully tested on U.S. Navy riverine patrol craft in the Mekong Delta in 1972. The Navy made further improvements to the weapon, resulting in the Mod 3 in 1976. The Mod 3 was adopted by the U.S Army in 1983 and remains in service to the present day.
The Mk 19 is a belt-fed, blowback-operated, air-cooled, crew-served, fully automatic weapon that is designed not to cook off. It fires 40 mm grenades at a cyclic rate of 325 to 375 rounds per minute, giving a practical rate of fire of 60 rounds per minute (rapid) and 40 rounds per minute (sustained). The weapon operates on the blowback principle, which uses the chamber pressure from each fired round to load and re-cock the weapon. The Mk 19 can launch its grenade at a maximum distance of 2,212 meters (2,419 yd), though its effective range to a point target is about 1,500 meters (1,600 yd), since the large rear leaf sight is only graduated as far. The nearest safe distance to launch the grenade is 310 meters in training and 75 meters in combat. Though the Mk 19 has a flash suppressor, it serves only to save the eyesight of its operator, not concealing the weapon's position. For night operation, a picatinny rail quadrant sight can be added for thermal and night vision optics.