The term was originally used to refer to the generation of machine guns which came into widespread use in World War I. These fired standard rifle cartridges such as the 7.92 Mauser, .303 British or 7.62×54mmR, but featured heavy construction, elaborate mountings, and water-cooling mechanisms that enabled long-range sustained automatic fire with excellent accuracy. However, these advantages came at the cost of being too cumbersome to move quickly, as well as requiring a crew of several soldiers to operate them. Thus, in this sense, the "heavy" aspect of the weapon referred to the weapon's bulk and ability to sustain fire, not the cartridge caliber. This class of weapons was best exemplified by the Maxim gun, invented by the American inventor Hiram Maxim. The Maxim was the most ubiquitous machine gun of World War I, variants of which were fielded simultaneously by three separate warring nations (Germany with the MG 08, Britain with the Vickers, and Russia with the PM M1910).