In reference to the M1917, it contained 7 pints of water. Which is 1.25 pounds per pint. Almost 9 pounds water weight compared to the air cooled M1919 machine gun. Pump is either active or passive circulation. The M1917 manual suggests mixing antifreeze when the passive water pump (convection) was no longer effective. Maintaining a water cooled system is much more intense. ie "Joe" will break it. http://www.big-red-one.org/FM 23-55 M1917 M1919 .30 CAL MACHINE GUN.pdf I don't completely discount a water cooled crew served machine gun in the future. Gels, convection, all sorts of technology are getting better every year. Even just a gel cooled neck pillow is something that would not have been thought of in the 1920s. We are in a time where air cooled is the best we have right now.