I was not aware that the .50 was fitted with a water jacket. The .30 yes, but not the .50. Any sources for info?
Ordnance Notes -- by Bob Stoner GMCM(SW) Ret.
AN/M-2 HB .50 Browning Machineguns
The venerable .50 Browning machinegun has been standard issue with the American armed forces since 1921. It has appeared in water-cooled anti-aircraft, air-cooled aircraft (flexible and fixed), and air-cooled
ground (heavy barrel) configurations. The AN/M2 gun is the direct descendent of the M1921. The receiver of the gun is the starting point for the above versions. In addition, the gun can be setup to feed from either the left or right sides and can be fired manually or electrically. The AN/M3 is a modification of the AN/M2 which will be described later. The gun can be setup to fire full automatic or both semi auto and full auto by the addition of some parts.
AN/M2 .50 BMG Water-cooled
The .50 BMG water-cooled resembles a scaled-up .30 BMG M1917A1. It uses a light weight barrel surrounded by a barrel jacket filled with a 60/40 mix of water and ethylene glycol. A packing on the rear of the barrel seals in the water/antifreeze mix at the rear and a packing at the muzzle end of the barrel (muzzle gland) seals in the water/antifreeze mix at the front. The barrel support in the trunnion block is brass, as is the front of the water jacket, to minimize corrosion . The water jacket itself is Parkerized (manganese phosphated). The water jacket is provided with two ports for connection of hoses and a condenser. As the gun fires continuously, the water reaches a point where it would boil away. However, the steam is routed through a condenser to cool it down until it becomes water again. The water is recirculated to the water jacket by convection. Weight of the gun with a full water jacket is 121 pounds. Because the .50 BMG is water-cooled, it can theoretically never overheat (as long as there is water in the jacket). Water-cooling prolongs the life of the barrel, especially when long, withering bursts of fire are the norm. As a result, the .50 BMG water-cooled was the primary light anti-aircraft gun until it was replaced by the 20mm Oerlikon in U.S. Naval service (1942). It was replaced in U.S. Army service by the AN/M2HB for anti-aircraft and ground tasks (mid-1942, early 1943). Most AN/M2 .50 BMG water-cooled guns were converted to either aircraft or ground guns as World War 2 progressed. The water-cooled type is obsolete and is likely to be encountered only in museums.
http://www.mst2-vietnam.info/Stoner_ordnance_notes/stonerhtml/AN-M2.50.htm