I've killed them with a 50 cal. muzzle loader, 30-30, 270, 22 mag and 22lr. The 22's were usuall head shots, but I killed more with the 22 mag and 50 cal than anything else. The place where I hunted them the most only allowed muzzleloaders, shotguns, and rimfire rifles. The 50 cal would do the trick, but when you get into a herd of them, one shot wasn't ideal. The 22 mag with CCI solid points would do the trick. The last one I took with it was a 300 pound boar. Good heart/lung shots with the mag worked well too. Do not use the Stingers or HP's on hogs because you do not get enough penetration.
Where legal I would stick to a deer caliber like the .270, .308, 30-06 or one of those.
Anytime you can take a head shot, you are better off, because even a good heart/lung shot often does not produce a good blood trail. The thick hide and fat layer has a way of sealing up and you just don't get good blood.
