No firsthand experience, but Ross Seyfried did a great article on them years ago, he said with a fairly long bullet, they kind of acted like a partition, very violent expansion, but if they hit anything hard, the soft lead sheared off, leaving the hard cast shank to penetrate. He had several samples of recovered bullets, and some had a large lead "donut", hole and all, with a wadcutter like shank of hard cast in a separate piece. He said they are a pain to make, but were a great choice for thin-skinned game.