The .50 was a scaled up .30-'06, or at least Browning said it was, and I think he should know.
But in the area of small arms, it is pretty awesome. It won't penetrate a modern tank, of course, but it will do serious damage to lighter armored vehicles.
Note: AP ammo does not penetrate armor through pure force, like a .22 punching a hole in a can. The bullet is stopped by the armor, causing a massive energy dump and melting the steel, allowing the AP core to penetrate. But to do that, the armor has to be thin enough that it doesn't act as a heat sink. If it does, the core will just stick in the armor as it "freezes". There was a German tank turret at Aberdeen with dozens of .30 AP cores sticking in it, sort of like a pincushion with sort pins.