I used to hunt down at Oakey Woods south of Macon. In the '30s, some fool imported 20 pairs of genuine Russian and turned them loose. They have interbred with feral hogs and the progeny have turned out pretty nasty. Anyway, I was trying to 'grunt' a buck in towards my stand (first time I ever tried that technique) and got some answering grunts, unfortunately, it wasn't a buck, but one big, hairy, mean, and mad male hog, which had many of the features of a Russian. Since it was close to the end of shooting light, and thinking that with the hog around, there would be no deer, I put the crosshairs on his chest where I thought the heart would be and squeezed the trigger. The hog staggered, but kept coming at me. So I put another one in his chest. This one dropped him, but when I got to him, he was still alive.
I was shooting a 30-06 with 165 Ballistic Tips. When we dressd him (and, boy!, did he stink!), we found then first bullet flattened against what seemed to be a layer of gristle about an inch under the skin. The second bullet had broken up, and part of it went about halfway into the heart. These are bullets that I have had go completely through the chest cavity of a full grown whitetail at 150 yards, expanding to about 2 diameters, from what I can tell.
So, I think that these hogs can be very, very tough.