The Zulu's had a mixed bag of varous european and africa firearms.
With the lack of availability of loaded cartridges, they went for firearms which could be easily loaded without tools.
At the time an Elephant gun, would be around .50 cal or .577 calibre, and normally doubled barrelled, as produced by Holland and Holland or Jeffery. The load was a whopping 100-120 grains of powder, normally in cartridge form. The secind barrel gave you a second chance that a single shot would not, especially if you had a four ton bull running at you.
The remedial .577 Enfields and various smoothbores they carried simply would explode if overloaded, unless as your anecdote says anything would go out pretty fast if it would go down the barrel, its a bit like the blunderbuss of C1780.
I fired a .50 calibre Jeffery double barrelled game rifle at our club, load was 120 grains, the owner told me the first round gives you a nosebleed, the second detaches your retina, I did one round, then wimped out..... and thats from someone whi will do 40 Martini rounds in a t-shirt.