A light and "cheap" version of the heavy machinegun, sacrificing water coolant and sustained fire capability for lightness and mobility.
The machinegun was first introduced during the steam era, but it did not become common and reliable until this time. Most machineguns were still water-cooled monstrosities intended for hours of accurate fire from static positions. The light machinegun changed all this, as it was mobile. Advancing troops could now set up fire positions of their own.