Ken S,
You have an 1865 Burnside Contract Spencer? Am I correct?
Question, Why do you want a .520 dia. mold?
You maybe able to find molds for the Smith or Maynard Carbines on the N-SSA Forums.
These usually cast bullets (depending on the lead/tin content) at about .515 dia. I think you will find this diameter fits your carbine better and will require a lot less sizing of the bullets.
My best,
Blair