Captain Jack (Updated: 7/23/2016):
Debuting well past the war in 1921 (the war had ended in 1918), the new machine gun was classified as a "heavy machine gun" and operated from the "short recoil" principle through a closed bolt function. It was initially a water-cooled weapon system which allowed for long-running bursts of fire and used to prevent the barrel from overheating (this obviously requiring a consistently cool water supply to be used). The weapon was chambered for the .50 BMG ("Browning Machine Gun") cartridge (otherwise known as the 12.7x99mm NATO in the post-WW2 world) and fed via an ammunition belt running through the upper receiver. The .50 BMG was itself a massive cartridge shaped like a traditional bullet and featuring a rimless bottleneck casing. It was also debuted in 1921 and attributed to the Winchester Repeating Arms Company, a firm which John Browning partnered with in the years prior to his collaboration with the Belgian firm of Fabrique Nationale. The weapon/cartridge combination went on to become one of the most lethal, ferocious weapon systems of all time, seeing use within dozens of national armies and irregular forces around the world through countless notable conflicts. With the water jacket and water-cooling system in place, the M2 weighed in at 121lbs and rate of fire was approximately 450 to 600 rounds per minute.
Once in operational service, the design was furthered to produce the improved "M1921A1" designation under the Colt brand (John Browning had died in Belgium in 1926, his existing work being carried on by others). It was not until the 1930s that a new Browning machine gun mark was released in the form of the "Browning M2" though this early form still utilized water-cooling for the barrel but instituted a new water circulation system along the barrel jacket. Large-scale Browning machine gun production was undertaken by Colt beginning in 1933. In the same decade (leading up to World War 2), an air-cooled variant was developed for use in aircraft and this, too, was confusingly designated as the "Browning M2". It would be this production form that would become the definitive entry in the Browning heavy machine gun line.
While the air-cooled version proved capable of firing the .50 BMG cartridge, it could not manage firing beyond 75 rounds before overheating the barrel to the point of fracture. An attempt to rectify the issue produced the M2HB ("Heavy Barrel") guise and this form was applicably given a stronger barrel assembly to help dissipate the inherent heat build-up. This made for a heavier weapon system (84lbs) but a weapon that could nonetheless be fired for longer periods of time. To help further relieve the barrel heating issue, a "quick change" function was added to the barrel assembly allowing an operator to replace the heated barrel with a cool one (this function came to be known as QCB - "Quick Change Barrel").
The M2 ultimately proliferated the American military inventory prior to and during World War 2. It was utilized in all manner of ways as a defensive and offensive offering. The type served in fixed and flexible mountings within fighter and bomber aircraft of the US Army Air Force (as the AN/M2) while also being the weapon of choice in combat vehicles including tanks. The machine gun functioned extremely effectively in the anti-aircraft/anti-armor role and could decimate personnel unfortunate enough to cross its firing path. Specialized vehicles mounting multiple Browning heavy machine guns in traversing turret mounts were produced as ad hoc anti-aircraft/anti-infantry measures as the war progressed. The weapon could further be implemented as an infantry fire support measure for suppression fire though this required multiple crew to manage its cumbersome operation (gunner, ammunition handler, transport crew). The M2 was further installed as an anti-aircraft measure on countless naval ships without loss of effectiveness. Range was out to 2,000 yards though targets could be reached as far out as 2,200 yards with some care (and sometimes a bit of luck). Muzzle velocity was rated at 2,900 feet per second, providing for excellent penetration values at range. Aircraft versions could achieve 800 to 1,200 rounds per minute.