This is why no one uses water-cooled machine guns anymore
It was a common sight in World War I – the image of a machine gun spewing bullets from a barrel protruding from what looks like a giant canister. That canister is a healthy indication that the barrel of the machine gun in question is being cooled by water in the canister and that the water will soon be as hot as the barrel.
For the troops who wanted to continue using the machine guns to keep wave after wave of enemy troops in their own trenches and not coming into yours and sweeping you all out with shotguns, this was going to become very important, very fast.
Water-cooled machine guns, as it turned out, required water to cool them. And the more you used them, the more they needed that water changed out. This may not have been terribly difficult during World War I, when movement along the front was restricted to just crossing no man's land into the next series of trenches a hundred yards away. Unfortunately for everyone's infantrymen, the water-cooled MG lived well past the Great War. So, the infantry were stuck hauling water for the guns, well into the Korean War.
But eventually, the guns were replaced with the less efficient but more manageable air-cooled guns. The problem with those was the barrels did not disburse heat quite as well as water-cooled weapons. So if a gunner isn't shooting in bursts to prevent overheating, there is a chance the barrel could melt or become deformed during use. The solution? Interchangeable barrels. Watch U.S. troops do it:
That video is sixteen seconds long. So as efficient as water cooling those barrels really is, asking machine gunners to carry the weapon, its ammo, and tanks of water on a journey to Mordor is just not as effective as swapping out the actual problem: the barrel.