The M1919A2, the predecessor to the M1919A4 and M1919A6, evolved from the Browning model M1917 water-cooled machine gun. The M1919 series .30 cal. machine gun was replaced by the M60 7.62mm machine gun.
M1919A4 was used as both a company level flexible light machine gun on the M2 tripod mount and as a fixed machine gun on armored vehicles. The M1919A4 had a heavier barrel with a ventilated barrel jacket, but developed a slower rate of fire (400-550 spm) than the water-cooled gun. Green Arrow Browning M1919A4 .30 Cal. machine gun (Copyright The Stackpole Co.).
M1919A5 was a modification to the M1919A4 for use as a tank machine gun.
M1919A6 was a war time modification to add tactical flexibility by substitution of a bipod in place of the tripod and addition of a shoulder stock and carrying handle. The M1919A6 had a lighter barrel than the M1919A4. It had a rate of 400-500 spm.
Information on this page courtesy of U.S. Army TACOM-Rock Island.