Automatic weapons have two basic methods of keeping the barrel cool: air and water. It doesn’t take very many rounds through a gun to get the barrel hot. Repeated and continuous fire will quickly get the barrel too hot to touch with the possibility of causing a serious burn if one is not careful. And, in the case of heavy machine guns, one can fire until the barrel starts to glow cherry red, then white hot to almost translucent where the barrel then literally melts down causing irreparable damage. Thus, by necessity, design and function, cooling is extremely important to the reliable operation of an automatic weapon.
Submachine guns are all air-cooled. Since they use pistol ammunition and are personal defense weapons rather than tactical sustained fire weapons, their primary means of cooling is ambient air. Design features such as firing from an open bolt allows air to circulate in the chamber area. Since the round is not seated in the chamber, a cook-off is avoided if the barrel is ultra hot. Sub guns also use twenty- or thirty- round clips requiring “down time” while reloading or replacing clips allowing the weapon to cool a bit between firings. Finally, some models have fins milled into the barrel. The purpose of these cooling fins is to increase the surface area of the barrel, thus allowing more of the barrel to be exposed to the air and more heat can be pulled out of the barrel. The point being is that these weapons, while certainly capable of getting extremely hot under heavy usage, were not designed for long sustained fire. Thus, ambient air is sufficient to cool the gun. The same basic design features hold true for machine rifles, such as the Lewis Gun, BAR and Benet-Mercie, of which they may or may not have cooling fins, even though they fire rifle ammunition. It is interesting to note that the Lewis Gun had an air-cooled radiator system which its inventor claimed sucked cool air in through its radiator casing.