An adapter device is provided for converting a grenade launcher into a weapon for firing shot shells. The adapter includes an elongate barrel member of a gauge for shot shell which is adapted to be received in, and extend through, the barrel of the host barrel assembly of the grenade launcher. A cap secures the barrel member in place in the host barrel assembly. A replacement barrel assembly for firing shot shells is also provided which includes a shell extractor for extracting shells from the replacement barrel.