I have always thought the traditional cape gun was the "poor man's" gun. Known as the "cape gun", because it was often the gun of choice of a Boer farmer in the Cape. Perhaps he had only one gun, and wanted a rifle for game and a shotgun for birds. The shotgun could also be loaded with big shot or a slug or ball. Usually it is a BP rifle and while the calibre might be large, as a BP it is still a medium game rifle. I don't necessary mean "poor" poor. If one is not very well off, buying ONE nice firearm may still be an extreme expense, so the average man might settle for one gun. To do it all. The common man did not have enough money to buy the fancy Rigby's, Purdeys, H&H etc usually.

Also one can only carry one gun. Properly anyway. So a shot and bullet firearm, very versatile when hunting for the pot, hoof and feather, on a walk in the veldt. Or the Indian jungle. Same as some still use a combination firearm!

Same, just side by side instead of under and over.

As for ammo supplies, could the owner not have loaded their own? Black powder would have been readily available. A cast lead ball or bullet could have been used in the .500 BPE. Lead casting equipment for bullets or balls was very common in the BP era.

What sort of resizing dies or equipment is needed? Or available? Certainly for smaller rounds, a hand vice or pliers type resizer might have been possible.

Or the owner simply ordered 200 rounds of .500 with the gun, thinking it might last his lifetime. And perhaps custom re-orders were available.

Edited to add: As Lancaster proves with a catalogue. Ammo was still available. Lots of older guns to provide for.