Machine guns were heavily used in World War I, and weapons of larger than rifle caliber began appearing on both sides of the conflict.
[1] Consequently, the American Expeditionary Force's commander General John J. Pershing asked for a larger caliber machine gun.
[2] Pershing asked the Army Ordnance Department to develop a machine gun with a caliber of at least 0.50 inches (12.7 mm) and a muzzle velocity of at least 2,700 feet per second (820 m/s).
[1] U.S. Col. John Henry Parker, commanding a machine gun school in France, observed the effectiveness of a French 11 mm (0.43 in) Gras incendiary round, the so-called balloon buster.
The Army Ordnance Department ordered eight experimental Colt machine guns rechambered for the French 11 mm cartridge.
[3] In order to adapt his machine gun to .50 cal Browning had to introduce a proper hydraulic recoil buffer instead of a cheaper stack of fiber disks on .30-cal M1917 and M1919 which he also tried on the M1921, as well as breech block cushioning.