Colin;
The 3600 grain balls came from the link in the very first post which included this;
This 2 bore double rifle regulation is for 500 grains of black powder and a 3600 grain round ball. A 3 inch group is considered a well regulated double rifle.
Watch Colin Stolzer explain and regulate his custom 2 Bore Jones Underlever Double Rifle.
I assumed this to be your rifle but by the time I posted had forgotten it specifically said Round ball. I believe the one I had seen about which was not a true 2-Bore diameter did use a conical "Ball".
As I said I fully appreciate what is involved in making such as this on a one-off basis & understand the Costs to do so.