As soon as the automatic type weapon appeared, there was also a problem with overheating. The designers are still working on this issue. If we talk about small arms, then it is quite difficult to do anything in this regard. The maximum that you can think of is to install an air casing like a PPSh, or, like Thompson's (submachine gun), to make the barrel a grooved, a kind of radiator. Things are a little easier with machine guns. The dimensions and weight of the weapon, namely the hand-held machine guns, are not so critical in this case. So here a simple solution to the problem suggests itself - attach a container for water to the barrel.
Water Cooled Weapon
Our most famous weapon of this type is the Maxim machine gun. The system worked well enough, the main thing is to change the water on time. But there was another problem - where to get this very water.
The Germans during the Second World War promoted the idea of using a machine gun, which would be both easel and manual. After the end of the war, other countries also became interested in a similar decision. In addition, over time, heavy machine guns became relevant only for use on armored vehicles, and in this case, water cooling, to put it mildly, is not very convenient.
As a result, in the case of machine guns, they decided to use removable barrels, which were quickly replaced by one another in turn. In our time, the issue of overheating is solved in a different way. In the conditions of modern warfare, firing is not provided for a long time.
It will be no less interesting and useful to find out for which the Germans in the First World War needed a mortar made of wooden planks.