@ ndnchf: Yes, the Rocket Ball has been replicated in modern times. I recall one of the gunsmith-reality shows on History Channel did just that in an episode some years ago.
They took a .41 caliber lead bullet, drilled a hole in the bottom of it, a little smaller diameter than a large pistol primer, and then made a recess that would serve as a primer pocket. This was all done on a lathe.
The cavity was filled with a pinch of blackpowder, then a large pistol primer plugged the cavity.
(Maybe the primer would need to be glued in place or secured with a layer of varnish?)
They made a handful of Rocket Balls this way, and they were fired in an original .41 cal. Navy Size Volcanic pistol. If I remember correctly the owner of the Volcanic pistol was some kind of celebrity, and the gunshop was a family-operated one in Colorado, where the owner piloted his own helicopter...?
YouTube had the answer: Rich Wyatt of the "American Guns" reality show. Couldn't find the Volcanic-episode, though...