What ever happened to water cooled machine guns.
I realize that they weigh substantially more than a modern air cooled MG but for a defensive fixed position that should not matter.
The last general use of watercooled MGs in US service was with the .30 caliber M1917 Browning watercooled, more accurate than the air-cooled guns due to the front barrel bearing surface at the front end necessitated by the need to keep the front gland packing watertight, but just loose enough to let the barrel cycle in recoil. The guns were used for overhead fire at basic training centers as trainees beneath crawled on the ground through the barbed wire as tracers flipped by overhead to the accompaniment of 1/4-pound demolition charges being set off in sandbagged pits. Both the added accuracy and ability to run 30 and 50-round bursts were features not possible with the air-cooled M1919A4 or M60, at that time, 1966 or '67. We used to refer to the MG firing detail- at night- as machineguns a-go go, with the fiddlers three, 3 guns being used to cover belt changes or possible stoppages, just like in the real world.