Can a brass frame revolver be converted to fire metallic cartridges?
Can it, yes. Should it be, no. If it is an antique, you would be
destroying its collector value to get a "shooter" that would
probably not be very good. If it is a modern reproduction, the BATF
would be after you for manufacturing a firearm because as a
percussion revolver it is not treated as a firearm, but once
converted it is.
The BATFE would not "be after you" for manufacturing a firearm.
You may do this, as LONG as the firearm is not "transferred" by
selling it. It must remain with the manufacturer....YOU for life.
If you decide to sell the firearm, it must then be assigned a
serial number and transferred legally through an FFL dealer.