I have shot many, many feral pigs using a 204 ruger, 223, 25-06, and 300 savage. And have seen them taken with a number of other substantially larger calibers. One thing I can say with certainty is the difference in the size of the wound between a 204 ruger and say, a 30-06 is negligable. Every pig I have ever shot in the right place died just the same using every caliber. The only pig I lost was with the 300 savage due to poor shot placement.
In regards to caliber, just about any centerfire rifle is suitable for taking medium game in my experience. The kicker is the bullet that is used. Bullets designed for varmint hunting can and do work, sometimes spectacularly do to their rapid expansion. The problem lies in their lack of penetration, limiting your shot options. Sturdier constructed bullets tend to penetrate better, but are usually less destructive.
The general rule should be to know your limits and select the rifle that you are the most confident with. A well placed shot using small, lightly constructed bullets will kill every time, where as a poor shot using heavy, tough bullets will often result in lost game. Larger, sturdy bullets don't really give any more room for error, at least not in my experience. They can provide you with a better opportunity to take shots other than the perfect broadside shot.
I'm not saying that small calibers work better or worse, not at all. I'm just trying to say that knowing how to shoot and ustilize the bullets construction effectively is what kills game.
