Quoting from FM 3-22.27:  "Although the MK 19 is a recent entry into the Army’s inventory, development began in 1963.  The first version was a hand-cranked, multiple grenade launcher called the MK 18.  In 1966 the need for more firepower inspired the development of a self-powered 40-mm machine gun called the MK 19, MOD 0.  This model was neither reliable nor safe enough for use as a military weapon system.  Product improvements begun in 1971 resulted in the 1972 MOD 1, of which only six were produced.  The MOD 1 performed effectively in Navy riverine patrol craft and broader applications for the MK 19 were found.  In 1973 the Navy developed the MOD 2, which featured improved reliability, safety, and maintainability.  In 1976 a complete redesign resulted in the MK 19, MOD 3, which the Army adopted in 1983.  The Army now uses the MK 19 within the tactical environment for defense, retrograde, patrolling, rear area security, urban operations, and special operations."

40 mm Mark 19 on USS Hopper DDG-70 in September 2006
Note the 25 mm Mark 38 in the background
U.S. Navy Photograph 060906-N-9851B-002

Operations Specialist 2nd Class Matthew Burton manning a Mark 96 combination 25 mm cannon and 40 mm grenade launcher on USS Chinook PC-9 in March 2003
Note the feed mechanism for the Mark 19 Grenade Launcher
U.S. Navy Photograph No. 030321-N-0000G-009

40 mm Mortar in October 2006
Note the empty shell cases
U.S. Navy Photograph No. 061009-N-4265O-004