From the ATF Website:
Because black powder firearms are considered antique firearms, the possession of a black powder firearm by a person subject to federal firearms disabilities is not prohibited by the GCA. However, a person subject to federal firearms disabilities may not receive and/or possess black powder firearms that can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof which are classified as "firearms."
Yep, and I don’t think Antiques are illegal to carry in Ca either...