Sorry, but this is nonsense. Sure, people will tell you that a .50 can easily penetrate an inch of steel...when fired at a short distance at near-vertical angle. But not a deformed bullet bouncing of the ground at a flat angle. And even then you would still need a smooth paved road to have a theoretical chance. German PzKpfw IV tank had a 10mm armored floor, for the Panther this was 15+mm and 25mm for the Tiger.
This is just another war tale, maybe it has some truth in it as one might have scored a lucky one-in-a-million hit somehow, but this would be just a morale boost for allied pilots rather than a practical way of dealing with a heavy tank.
If there's a war story with that legendary Tiger tank in it, and someone blowing it up easily, you should be very sceptical...
Have a look at this, this MG-vs-tank thing has been discussed a lot.
http://www.warbirdsforum.com/topic/759-duel-p-47-thunderbolt-vs-tiger-tank/