I don't think there is any, it's just a marketing term since it is offered by the people who sell HeviShot. The product description states: 100% nontoxic bismuth pellets. They also state: "Our new HEVI-Bismuth™ Waterfowl shells utilize bismuth, which is 22% denser than steel, to deliver more downrange lethality at an affordable price below that of our tungsten-based loads. And, it’s safe for older fixed choke and fine classic doubles shotguns." Just marketing.

Bruce