Many 1853, 1858 and 1861 Enfield percussion military rifles in .577's were converted to .577 Snider simply by removing a section of the barrel at the breech, threading it and screwing the barrel into the snider patent action. The original stock, with further inletting, was kept.

I suspect this is also how the Zulu shotguns came to exist - by removing and threading a section of the breech of a percussion shotgun, threading it and screwing that into what appears to be snider-type action.

Of course, other percussion guns were converted as well, here and abroad.