http://www.p47advocates.com/messages/464.html The crucial excerpt:
I've often wondered about reports of "bouncing bullets off the road into the bellies of tanks to set them afire"-- as the bellies of these tanks are still too much for such light projectiles to handle, particularly after losing much energy by striking the road surface first, and then likely tumbling or yawing severely before strking the belly armor. However, having been a tank officer for several years, and having armor-antiarmor study as part of my civil job for many years, I think that the answer may be that these stories ARE TRUE-- but not due to the penetration of the belly of the tanks, but due to strikes against the "drain plugs" and access plates (or places where they should have been installed) beneath the tanks. All tanks have to have these (to let spilled oil, fuel, oily engine "slobber", and of course, rainwater and melted snow drain away, or to gain access to the underside of the powerplant and other machinery inside the tank. Tanks of all types are notorious for having lots of oily, fuel-soaked, greasy gunk all over the bottom of the engine compartment, and they're also widely known for having "missing" drain plugs and access plates-- at any given moment, a surprising percentage of the world's armored vehicles are driving around with one, several or many of their drain plugs and access plates missing, even today. In combat, with hasty, midnight servicing and repairs always being made, often in deplorable muddy, slushy ground, or in the churned up grass and turf of hasty assembly areasa, it's only natural that small drain plugs, and access plates (typically 1.5" to 4" in diameter) are lost, or come loose, or are purposely "lost" or left "open" by the crew. I think that the basis for these type reports may be just that-- the P-47 pilots were bouncing rounds right into these areas of the tank, and vs. the Germans' gasoline-fueled vehicles, a single burning tracer or burning fragments from an API strike would easily ignite the spilled or dripped grease, fuel, oil, dried fragments of vegetation, greasy residue, and all the other gunk that coats the bottom of the engine compartments of tanks in combat. They weren't penetrating the belly of the tank, but it probably worked well enough to keep on doing it.