The frontal shot "right between the eyes" is a bad shot on a hog, the slight downward angle of the bullet (if your close) will end up only going into the hogs sinus cavity. Also the slope of a hogs head tends to make the bullet skip up and not penetrate the skull at all. If you are close, like shooting one in a trap or pen, draw a imaginary line between his eyes, then aim half way between there and the top of his head. The side of the head 1/2 way between the eye and ear is your best shot, any hit in this area is like turning off a switch. A .22 long rifle will kill the biggest hog if you hit him here, but I don't suggest it as it can bounce back and hit you.
If you are going to eat your pork, I would suggest NOT head shooting them, as they don't bleed much. If you can get to them quick and stick them, then that will be ok. To stick a hog, find the soft spot right above where their chest comes to a point. Stick your knife straight in, turn it 90 degrees, make a couple of quick cuts and pull back, you should get a stream of blood the size of the hole.