Thanks everyone.
Uncle - HA! What a coincidence. I decided to try the new Remington Premier Tips. They grouped really well at the range. These are the ONLY slugs that this gun likes out of 5 brands I've tried. Well, the Remington Copper Solids grouped okay too. As you can see from the pic, they don't leave the golfball-sized hole that some slugs do (e.g. Lightfields), but man, he was devastated on the inside. A few bones were utterly demolished. What's interesting is that you can see both the entry and exits wounds in the pics above. No gaping holes whatsoever.
Hey, I think you're the guy that helped me a couple of weeks ago with my sighting in problem. Thanks again!