Shotguns are not ideal for shooting hogs unless you don’t care if you recover the hogs. It will kill them but they don’t leave a good blood trail. As far as rifles go a good headshot with anything from a .22 rifle up will drop them.

Shot placement is key on a hog head when using small calibers or non expanding bullets. From the eye forward and from a little below the eye down is gonna be a long slow death. I personally try to ear hole them