

A water-cooled barrel improves the sustained fire ability of a machine gun by boiling off water into steam. This system prevents the temperature of the barrel from exceeding the 100° C boiling point of water. This keeps the barrel cool enough to prevent getting deformed. This comes at the cost of adding significant weight to the weapon.
Air-cooled barrels are lighter but also heat up significantly quicker and can get hot enough to melt. To overcome the overheating issue, modern GPMGs have quick change barrels for continuous firepower. Some modern fixed machine guns such as the M134 Minigun feature multiple rotating barrels.
The photo is a Fedorov-Degtyaryov LMG with a “Maxim-type” water-cooled barrel.