In this video Taofledermaus shows these slugs he got from Germany in June and he has been trying to figure out a safe yet exciting way to demonstrate them.
These were designed to fit a 9mm round inside them. Without a round, they weigh 1.5 times the weight of a 1oz. slug.
This required a carefully researched powder load. These were fun slugs because they were relatively accurate and the fragmentation and power was unexpected and giggle-inducing.
These were meant to be shot through either a rifled choke or full rifling to obtain spin-stabilization, but because he was leery about the tolerances, he shot them through a cylinder-bore for safety.