An associate of mine who owns a (eeevillll!) .50 BMG decided to create his own completely unofficial and unscientific penetration test. So he built this:
This is a simple wooden frame which, he says, he packed with "1800 sheets of .02 inch thick card board. That made a 3 foot deep target." Here's another picture:
He fired three shots into it at a distance of about 125 yards:
He reports that the projectiles penetrated from 13 to 15 inches. For comparison, he also fired some rounds from his 9mm into the "medium." I didn't ask what range, but I'm sure it was typical close pistol range, 10 to 15 yards or so. He reports that the 9mm bullets penetrated about 3 inches.
And what would such a test be without pictures of the recovered bullets? No 9mm bullets, but here are the recovered .50 projectiles:
This is the kind of thing that happens in Wilson County when we go down to the creek and can't find any turtles to shoot.