We used to hunt feral hogs on 7 mile island in the TN river, way back when I was young and REALLY crazy. Some Auburn grad had imported some asiatic stock including some asiatic wild boar genes, and some of those bad boys went over 400lbs. One of my friends shot one of them in the chest with a .44mag, and it didn't penetrate the chest gristle, or even slow him down much. Trees didn't do much good, because all the timber on that island had been cut, and all that had grown back was saplings, and even if you got up one, the hogs could push it over if you didn't kill a bunch of them first, which is fun while you're hanging on to the thrashing tree and trying to aim. My weapon of choice was the 10ga goose gun with 00buck or #1 buck.

BTW, shotgun for squirrel?? .22 shorts, or rat shot .22 shells if you don't mind breaking teeth. Or use something huge and shoot the limb under the squirrel, and the transmitted shock snaps their necks and they fall out of the tree without holes spoiling the meat. I've killed them that way with a Savage o/u 22mag/20ga with the shotgun slug, after I missed with the .22mag...