Do not take a head shot with a .22 LR or even the .22 mag if they are facing you.
Side head shot. Draw a line from the ear hole, thru the center of the eye. Put the bullet between the the ear and eye on that plane. You BETTER be able to hit a nickel 100% of the time at what ever distance you're shooting, cuz that's how much error you have. Back of the head works well.
Get a .22 mag if you're serious about doing this. Winchester Super-X 40 grn solids....