Six shot, "revolver" type 40mm grenade launchers have been around for years. But the U.S. military has never adopted them, until now. The U.S. Marine Corps has, after several months of testing, issued these weapons to troops in Iraq.
The MGL-140 40mm, six shot, grenade launcher is now called the M32. This weapon is, literally, a shotgun size revolver that fires standard American 40mm grenades. Thus it has a minimum range of 30 meters, and a maximum range of 400 meters. The weapon is 32 inches long, and weighs 13.2 pounds empty, and 20.3 pounds loaded (40mm rounds weigh about 19 ounces each.) Like any 40mm grenade launcher, it can fire lethal and non-lethal (tear gas, Etc.) rounds, and fire all six of them in a few seconds. The M32 also has a rail for mounting various types of aiming devices (day and night scopes). M32s retail for $6,000 each, but the marines have gotten a volume discount.
Currently, each battalion has one M32, to be used to see how the weapon performs in combat. If user reports are positive, more will be issued. Marines and soldiers have used their single shot, 40mm grenade launchers, a lot in Iraq, and with much success. But the single shot 40mm launchers have been around since the 1960s, and the marines wanted to try something new. Weapons like the MGL-140 have been used by police and military organizations for years.