I'm going to be truculent and disagree with you lesofprimus. To me the concept is soundly in the realm of the impossible. I have never seen any post-battle research that indicates that .50calibre rounds killed a tank by bouncing up into its undersides. This is in the same leauge as the claims of P-47s turning over Tiger tanks with concentrated fire.
I don't believe that a .50 calibre round, or even a steady stream of .50 cal round,s could do serious damage to a German tank after being deflected off the ground and into its belly. Dispersion and ground deflection alone are going to gaurentee that the rounds wont line up neatly and punch through the armour.
At a minimum the rounds have to pass through 26mm of armour, at an unfavourable angle after coming up off the ground. The .50 cal is a good gun, firing a good round, but the physics of it is simply impossible. A 12.7mm round doesn't have a desirable length/diameter ratio to penetrate armour that more than twice its own diameter. The energy it would expend trying to punch through that much armour would rob it of all velocity and stop any spalling effects, which is really the most leathal thing to a tank crew.
Think about it. If it hits the ground at 30 degree angle and then leaves it at a similar angle, there has to be a serious loss of energy to alter the direction of travel. So your round traveling at 30 degrees now hits the flat bottom of a tank, with anywhere between 1 and 3 1/2 inches of RHA.
Assume that the bottom armour is just 1 inch. A round hitting flat (90 degrees) has to punch through 25.4 mm of armour, better than the best performance of WW2 US .50cal AP rounds by about 2mm. If the round strikes at ~60 degrees, it has to pass through approximately 30mm of armour. If it strikes at ~45 degrees then it has to pass through ~35mm of armour. It it strikes at ~30 degrees then it has to pass through about 50mm, or twice the value of the armour at 90 degrees.
What I do accept is that many German tanks were disabled by straffing. Pouring rounds into the engine exhaust grates, setting off external fuel and ammunition stores, damaging tracks, damaging crew doors, damaging external equipment ect would all contribute to a 'soft kill' i.e. the tank is abandoned or no longer in fighting condition, but is capable of leaving the battle-field and is fully repairable. German tank crews who underwent air attack with aircraft firing HMG rounds describe it as something similar to standing in a corrugated iron shed and having someone hurl a handful of pebbles at it.
Just a hypothetical to consider; if the .50 WAS indeed capable of killing a tank through this method, why wasn't it used by other airforces, with heavier standard weapons? Surely the British with 4 20mm Hispanos on the Hurricane, Typhoon and Tempest would of done something similar? The Hispano, with a similar M/V and 3 times the round weight, would surely be better at similar straffing atack.