I believe that a .500" bullet will fit right into a "sabot". Something heavy for a 500 S&W should work. Check out Ballistic Products for more convential slugs. Lee Precision and Lyman make molds if you cast and the 525 grain Lyman slug is a winner. Here is link for sabot from Ballistic Broducts
http://www.ballisticproducts.com/RSS...ctinfo/322RSS/