Bullets of all calibers do strange things.
This is an old thread with lots of good info, especially from drgrant, well said. Since it's been kicked I'll add my two cents.... All skulls are not created equal, and I have heard of a .45 ACP going around the victim's skull without penetrating. The skull, especially the forehead and the rear of the skull, are extremely tough, made even more so by the smooth curved surface. The sides of the skull, though, are flat and not as thick as the front and rear aspects, so it's a LOT easier to punch through there. This again is why I feel shot placement is critical, if you can get them in the "triangle" - from the tip of the nose extending out to the eyes, you will increase your chances of penetrating past bone and hitting the medulla. But, 98 times out of 100 -- at close range, with ANY bullet profile, and even with frontal or occipital bones, a perpendicular strike WILL and almost MUST shatter bone and penetrate. The hole may be neat, but shatter the bone it still does.