I would highly recommend a .22 to the head. Very effective, and no mess. Shoot from behind, right in the back of the head, or just stick it right in the ear. I would not recommend center fire rifle, or shotgun slug to head. Very messy, eyes popping out, jaws disintegrating, brains splattering..... If your main gun is all you've got and a second shot is absolutely necessary, get a broadside angle and shoot right squarely through the rib cage. In WI, it's legal to carry a handgun while deer hunting. Technically not legal to use the .22 for deer hunting, but I've used mine for dispaches a few times.