The Mk18 Mod0 grenade launchers was developed by the Honeywell corporation in 1962, and was the first weapon in what would became a category of high volume grenade launchers used by the US military. The modern iterations are all self-loading, but this first example was fired by a manual crank handle, like a Gatling gun. The Mk18 used the same 40x46mm grenade cartridge as the single shot M79 launcher, and this round's low pressure allowed the Mk18 to use a rather unusual breech mechanism.
Unlike most belt-fed weapons, the cartridges in the Mk18 never left the belt. Instead, the breech consisted of two rotating spindles which would form the top and bottom halves of the chamber, closing around each shell as the handle was cranked. As a result, a loaded belt of grenades fed into the weapon, and a belt of empty cases came out the other side. Another effect of the low pressure cartridge was a rather short effective range, which limited adopted of the weapon to the US Navy, which bought 1200 and used them primarily on riverine patrol boats. In this application, the short effective range was not much of a hindrance, and the volume of high explosive firepower was a significant asset.
Armament Research Services (ARES) is a specialist technical intelligence consultancy, offering expertise and analysis to a range of government and non-government entities in the arms and munitions field. For detailed photos of this very cool early grenade launcher, don't miss the ARES companion blog post:
Cool Forgotten Weapons merch! http://shop.bbtv.com/collections/forgotten-weapons