Before the end of World War I, the U.S. Ordnance Department recognized that water-cooled machine guns took up too much space inside a tank. Consequently, the water-cooled M1917 was converted to an air-cooled model by surrounding the barrel with a perforated metal jacket.
As World War II approached, the Ordnance Department was committed to developing an air-cooled machine gun for infantry use. The result was the M1919A4.
At 41 lbs for gun and tripod, the M1919A4 was much lighter than the water-cooled M1917A1 (93 lbs for gun and tripod). On the other hand, the air-cooled weapon was unable to maintain the same level of sustained fire as the water-cooled M1917A1, and did not have the steadiness of accuracy as the heavier weapon. But its light weight and ease of set-up made it much more useful as an offensive weapon than the water-cooled guns.
In fixed defensive positions, however, the water-cooled M1917A1 saw much use in Korea. With anti-freeze in the water jacket, the heavy MG was more reliable in intense Chosin cold, as was particularly observed in the savage Reservoir battles. In any weather, the heavy was also more stable and, under intense attack, its greater sustained volume of fire was much appreciated.
Moreover, the A4 was crticized for slowness of set-up and vulnerability of crew. To meet these weaknesses, the M1919A6 was developed, and saw use in WWII, Korea and Vietnam.
