With a few modifications, this weapon uses pre-loaded explosives bathed in blood and so it needs no ammunition. It shoots five explosives at the same time and causes considerable damage to anyone standing near the impact point, even the player.
To find this, you must leap into the dark void just to the east of the last savegame terminal near the level's exit. You will not die and you must continue straight on towards of a destroyed bridge. A little farther from it, kill all ghouls and loot the footlocker toward the ice cream truck.