BTW what was the M2 machinegun's original intended use?
If I understand my history right, when the M2 Browning Machine Gun and the .50 caliber Browning Machine Gun cartridge came out in WWI, it was designed for use against tanks, which were a fairly new threat at the time as well, and it was moderately effective in this role. However, armor evolved rather quickly and by WWII, the .50 cal remained effective against light armored vehicles, aircraft, and personnel, but not so much as an anti-tank round.
The .50 offers enough mass to be employed with explosive compounds, but these don't explode in the common understanding of the term. They aren't like grenades. It is more of an incendiary effect used to make the round more effective against armor than purely kinetic energy and momentum would normally allow.
Here's a video from a demonstration for the Marine Corp Scout Sniper program on the M82 SASR:
http://www.youtube.com/watch?v=i9lMViBr6d8
When you get to about 5:45, it starts showing the actual effect of a modern armor-piercing incendiary round. The round is described earlier, right after the introduction of the rifle.
Performance of ammunition in the skies above Europe and the Pacific during WWII would have been quite a bit worse, but still impressive to anyone on the receiving end. It still would have been terrifying to watch your Zero disintegrate around you in a hail of gunfire.