Bear Grylls used a 5 inch combat knife to take one once, I believe in the badlands of Montana or something like that. Personally, I would use my .308 to take one, myself. wouldn't want to get too close to a Feral Hog.
I remember seeing that episode. He took a smaller pig that was tied up to a tree IIRC. Now me, a 5.56mm will work with right placement, 7.62x39mm or .30-30 is a lot better, .308 is grand, and anything larger will work too.
