When developed, the .50 BMG would have penetrated the armor of WW1 tanks. By WW2, it might have penetrated the armor of a Panzer I, which was not in use by the time the US entered the war. It also might have penetrated the armor of a Japanese Type 95 light tank. But against unarmored vehicles and enemy infantry ...
The .50 BMG was also probably the best compromise between ammo carried and hitting power for fighter aircraft. The .30, .303, or 7.7 mm allowed more ammunition, but its hitting power was not great. The 20 mm cannon had great hitting power, but the amount of shells carried was not great. Too hot, too cold, but the .50 BMG was just right, for WW2.