Wikipedia:
The Vickers machine gun or Vickers gun is a name primarily used to refer to the water-cooled .303 British (7.7 mm) machine gun produced by Vickers Limited, originally for the British Army. The machine gun typically required a six to eight-man team to operate: one fired, one fed the ammunition, the rest helped to carry the weapon, its ammunition and spare parts.[1] It was in service from before the First World War until the 1960s, with air-cooled versions of it on many Allied World War I fighter aircraft.
The weapon had a reputation for great solidity and reliability. Ian V. Hogg, in Weapons & War Machines, describes an action that took place in August 1916, during which the British 100th Company of the Machine Gun Corps fired their ten Vickers guns continuously for twelve hours. Using 100 barrels, they fired a million rounds without a failure. "It was this absolute foolproof reliability which endeared the Vickers to every British soldier who ever fired one."[2]

Note the wrap on the barrel of the Mk 1. Wool on the inside and leather or canvas on the outside. One of the means of cooling the barrel was to saturate the wool and put on this wrap.