I read or saw, way back when non-toxic shot made of bismuth were allowed, that the test on bismuth pellets was to crush a pellet in needlenose pliers. The pellets would break and crumble somewhat, with a multitude of cracks around the perifery of the edges as the pellet squashed. Perhaps more tin is needed in the melt for a round ball to maintain it's integrity if used for hunting, to keep the ball from breaking?
Does casting bismuth in your lead pot contaminate the pot for lead, similar to trying to cast lead with zinc in it?