Some guys I used to work with were using nickel or copper plated BB's and said they dropped in their tracks. Problem is they are hard to obtain unless you reload. #4 buck works well though, I can tell you that. But if it is just for bobcat I think a #4 bird in lead or, like Ladobe said, Hevi-shot. Apparently #4 bird lead, basic field load, works well on raccoons, so a bobcat it should work just fine I would think considering the size and weight of your mentioned game.