To my mind, the undesiputed king of machine guns, and a significant factor in US success on the battlefield in WW II, and ever since.
However, the M2 is a heavy machinegun. Its in a whole different class from the 1919 Browning and the MG 42. Now, while the Germans did use the MG 42 as both the light and heavy machinegun roles, the gun is not a heavy machinegun. It is best described as a GPMG, a general purpose machinegun.
And that is something the M2 definately is not. IN infantry configuration, the M2 reciever alone weighs 84lbs! The barrel is another 23, and the tripod about 65lbs. Considering each round of .50BMG fires a bullet that weighs about 6 times that of a .30-06, and at a slightly higher speed, all that weight is needed! But you pay a price in mobility. Weight vs mobility is why ground mount use of the M2 is rare. We mounted Ma Deuce on every kind of vehicle capable of taking it, on the ground, and in the air, for mobility.
True the BAR, Thompson, Sten, PPSH, MP40 etc are legally machine guns. SO is the M16. IF it fires full auto, then legally its a machinegun, be it tripod mounded belt fed, rifle, carbine or pistol.
That's how the ATF and the law define it. However, military defintions (and the terms in common use by civilians discussing these arms) are a bit different. Automatic rifle, light MG, Submachinegun, and assault rifle are better descriptors of the arms, by both their design and tactical use.
All else being equal (and it almost never is) bigger bullets tend to work better.