They do have sloped skulls that are pretty good at deflecting bullets. It's not terribly unusual to find one with a substantial scar on the forehead from a glancing bullet. I skipped a 110gr barnes 300BLK off one years ago, although a shot perpendicular to the forehead will definitely penetrate, as Gstew pointed out.I had a "guide" tell me he seen a hog take a 45-70 straight to the head and shake it off and run away. He reccomended large 30 cals minimum. Absolutely no 223 6mm 6.5 etc. I lied and said I had one, shot 2 with my 6.5 prc and absolutely annihilated them. I wish I would have used a smaller gun due to the amount of meat damage. I went in totally confident but it was still wild to see these "tough" animals die in half a second.
As usual, shot placement above all.