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This Pulemyot Maxima PM1910/30, is one of those really oldschool machineguns, a Russian licensed build of the venerable Maxim design.
What you have is a short-recoil action with a toggle-lock, so mechanically these guns actually have a lot in common with the fabled Luger pistol, just belt-fed and scaled up for big and powerful rifle cartridges, this one being Russian is in the 7.62x54mmR Nagant cartridge, naturally, and being the 1930s upgrade, its water cooling jacket has a big lid on top, the idea being that in an emergency, you can take a bunch of snow off the ground and shovel it into the jacket and it'll help cool the gun.
The idea with these old machineguns is that since they can fire as long as you hold down the trigger, they should obviously be capable of firing for all of eternity, thus they feed from belts, either canvas ones or linked metal ones, depending on model, and they have a thin barrel that's submerged in a tank of water. As the barrel gets hot as fuck from firing, the water absorbs that heat, so what happens is that the barrel itself never gets hotter than the boiling point of water, as long as there IS water, and that's actually not very hot at all for a firearm, thus you don't have to worry about premature wear or loaded cartridges cooking off. This means if you have a supply of water, a supply of ammunition in belts, you can keep firing and firing until you run out of of one of them, it will never overheat.
There were setups for recapturing some of the evaporated water as it turned to steam (a rubber hose piping the steam to a jerrycan where it can recondense), and if really needed, the crew could use their piss, with the smelly consequences that entailed. These guns were also built strong to handle this volume of fire, so they basically never ever wore out, therefore these kinds of guns were also VERY expensive.
Russia actually kept making these until 1945, and many armed forces kept them around for a while after WW2.