I'm a bit confused. You said this gun was a rolling block originally barreled as a 28 ga. shotgun. All you said you had your gunsmith do was to rifle the barrel. But you are attempting to chamber 24 ga. brass cases. Did he also ream the chamber out to 24 ga.?
I think I'd start by remelting your chamber cast and getting accurate measurements after the specified time for expansion of the cast. You could probably upset or expand the .562" round balls you have enough to push them through the bore to get an accurate bore and groove diameter.
