Lol, just about any round you use for a head shot is going to be effective. I have done in big pigs with a .22 short with a head shot.Just for amusement, I tried some of my preferred home defense 5.56 ammo, Hornady's 62 gr. TAP® Barrier™, on a couple hogs. Head shots at 75 yards were absolutely devastating. I was surprised, and pleased, with the damage that ammo inflicted.
I am also not one of those guys that says .223 is not enough gun for pigs( I prefer something in a .30 caliber, but I like watching them drop in their tracks) as I have seen it kill plenty of pigs.
I have also spent more hours looking for pigs shot with .223 than with .308.