A modified version of the M79 Grenade Launcher, commonly known as the 'Pirate Gun.' With its sawn-off butt-stock, short barrel, and a Docter Optic sight the weapon certainly resembles a pirate's flintlock
M79 grenade launcher - Wikipedia
M79 Grenade Launcher - MACV-SOG: A Unit of Modern Forces Living History Group
Nuprol N79 Short 40mm Grenade Launcher
King Arms Full Metal M79 Airsoft Grenade Launcher (Type: Sawed Off / Real Wood) - AirsoftGoGo
King Arms] M79 SAWED-Off Grenade Launcher[Short Ver.] – SIXmm (6mm)