Based on the M1919 machine gun, the M2 was the big brother. The cartridge and the gun were designed simultaneously. It's another Browning design, although the finished product required work that was completed after JMB's death. The result was a machine that has been in continuous service since it's inception and is in use by the U.S. military today. My son rode behind one in the turret of an MRAP in Afghanistan. Those young men in the picture above were behind the same gun 70 years ago.
The history, use, and variants can, and have, filled volumes. But the basics look like this.