Another great picture from a vet’s personal collection on the SOG group. This one of the MK 18 MOD 0 grenade launcher. The Mk18 is pretty neat just because it was hand crank operated like a Gatling gun. It was belt fed and fired 40mm grenades and was used on boats and reportedly in fixed positions, I have no idea how common that was though. As you can probably guess it was replaced by the MK19 grenade launcher.
Some numbers on the MK18 say it had an effective range of slightly over 2,000 yards and a rate of fire of 250 rounds per minute. That probably means sustained rate of fire without over heating etc.