"I think the semiauto .22LR is most common and if you get a good close up shot and can place it in the head it's fine"
I often hunt on a military reservation where a shotgun or .22 is mandatory, so i hunt with my Ruger 10/.22. Will not take a shot over 50 yards and limit myself to head shots only. Most shots required to kill a hog was 7. If you hit a hog between the eyes or near the ear he will almost always go down: If he is not dead just keep shooting.