Do you have an idea when the gun was made?
Based upon weight only, I would say it is a 5 or 6 dram gun. You need groove diameter and cast the round balls slightly over. Going to need brass cases as I don't believe a ball that large will go into a paper/plastic case and chamber in your gun. Brass cases will have to be made to fit your chamber and the mouth of the case sized to accept your ball/bullet.