Just thinking about this a little bit. In the revolutionary period days, fused shells came in to use for artillery. The problem would be, how to make them? In the winter time here in the north country we make hollow ice globes with water balloons. Fill them with water, place outside, and an appropriate wait later bring the balloon inside. The water on the outside of the balloon has frozen, but the inside is still liquid. It's a small matter to just drill a hole through the balloon and ice through to the core and drain out the water that remains. You then have your hollow ice globe. Would hollowed out cannon ball shells have been made in a similar fashion? Cast the molten iron, and then take the mould apart while the interior metal is still molten, and drain it away?. The hole could then be threaded for a fused plug.