I found this online:
While it's sinfully expensive, bismuth shot has some great qualities. It's an alloy of 97 percent bismuth and three percent tin. When examined, it looks like little silver ball bearings. Bismuth has a specific gravity of 9.7, which is midway between lead at 11.3 and iron (steel shot) at 7.
Being denser than equivalently sized steel shot, bismuth retains more energy as range increases. With hard lead shot having a Brinell hardness of 11, bismuth measures about 18 while steel shot registers approximately 110. Being relatively soft and malleable, bismuth can be safely fired in "presteel shot" barrels and chokes.
This is perhaps bismuth's most endearing quality -- permitting us to continue to shoot our Foxs, Parkers, L.C. Smiths, Brownings, Winchesters, Remingtons and other fine shotguns without concern and in full compliance with nontoxic shot regulations.
If there is a downside -- other than cost -- it is that bismuth shot is somewhat brittle. This has lead to the development of two different schools of thought when it comes to handloading bismuth.
Ballistic Products, Inc., which markets bismuth shot to handloaders, recommends adding a bismuth-specific buffer to the shot column and adding a felt or cork cushioning wad to a long-bodied shotcup wad. Their idea is to protect the bismuth shot as much as possible during initial setback and during its journey down the bore.