When I had my Sharps I got a mold from Eras Gone By. The bullets miked .549 on the driving band and the base was .530. I only shot it few a times before trading it off so I can't pass judgment on the bullet.
Breech loading externally capped pre 1865 firearms are permitted. Examples are Sharps, Smith, Burnside, Jenks, Hall, Gwyn & Campbell, Gallager, Joslyn, Maynard, Merrill and a lot more.