I just though cast iron when hit hard has a tendancy to splinter when hit hard, which is why bomb cases are made of it.

I've seen some very silly things done with it, such as making improptu firework mortar tubes out of it, or using a (cast iron) fencing mell with (steel) splitting wedges. It's amazing how loudly you can shout at people to stop what they're doing, from behind a solid object.