In South Africa, I had the chance to fire a couple of original Lee-Metford black-powder cartridges, made (IIRC) in the 1880's. Some of them didn't fire because the primers were dud, but those that did work went off with a perfectly satisfactory bang. We extracted the bullets from those rounds with dud primers, and inspected the powder, which looked OK: so we loaded the powder into a modern case with modern primer, re-inserted the bullet, and they all went off just fine too. So, BP more than a century old, having gone through who-knows-what in the way of storage, still worked as intended...