|The following is based on Fallout 3 cut content and has not been confirmed by canon sources.|
The grenade launcher is a cut content weapon in Broken Steel add-on for Fallout 3.
The grenade launcher is listed in the G.E.C.K. but was not placed in game. It's a modified combat shotgun that shoots grenades instead of shotgun shells.
This weapon was created during one of Bethesda's tutorials on how to use the GECK and appears to be left in the game.
The grenade launcher can fire a total of about 145 rounds, the equivalent of 49 reloads, from full condition before breaking
This item only exists in the game files, and cannot be obtained without console commands on the PC.