The U.S. is slowly replacing its 1960s era Mk19 40mm grenade launchers with a lighter and more effective Mk47 model. The replacement program is moving slowly, with the army still ordering Mk19s (6,000 new ones are being manufactured), even while several thousand Mk47s are being introduced. The Mk47 is lighter (40 pounds) than the 77 pound Mk19. It is a little more compact, and has the electronic sighting and arming system which enables the gunner to determine the distance of the target (with a laser range finder), then push a button to have the weapon electronically program each shell, just before it is fired, for the number of seconds until the shell will explode, at the proper range from the gun, and above the enemy troops. If the time fuze does not work, the shell explodes when it hits something.
Both these 40mm weapons have a max range of 2,000 meters (and an effective range of about 1,500 meters). The Mk19 weapon, with a tripod, weigh 136 pounds, while each 48 round magazine weighs 60 pounds. The Mk19 effective rate of fire is about one round per second, and is usually fired in short (a few rounds) bursts of these 19 ounce grenades (which kill or incapacitate most people with six meters of the explosion and can wound at twice that). The Mk19 is more complex than your usual machine-gun, expensive (about $20,000 each) and jams more frequently (once every thousand rounds, compared to once every 10,000 rounds for the M2 12.7mm machine-gun.). But it is reliable enough to remain popular and in demand. The lighter Mk47 costs about 50 percent more, and is about as reliable as the Mk19.
The U.S. Navy developed the Mk19 in the 1970s, for use on river and coastal patrol boats. The army adopted the weapon in the early 1980s, but it wasn't until the 1991 Gulf War that the Mk19 saw much action. Or at least some action. Users noted that the Mk19 was very effective out in the open, not so much in urban areas. In 2003, when the army and marines encountered much more combat out in the open, they found the Mk19 to be very valuable, more so than machine-guns.
What really makes the Mk47 different is the Mk285 40mm shells, each equipped with a time fuze. Normally only used in larger artillery shells, a time fuze in 40mm grenades makes it easier to get at enemy troops on roof tops or behind walls. These shells, which cost $213 each, can only be used with the Mk47. This first troops to get the Mk47 were U.S. NAVY SEALS, and other SOCOM troops.