No real knowledge on this myself so take this with a pinch of salt.
A friend said he was given a load of assorted loose eley paper case cartridges from way back when, some are nitro, some are black, it was his thought that the nitro had degraded? ( i don't even know if this is possible let alone true ) and that all the cartridges were really slow shooting but with enough lead they still brought pheasants down.
As i say no experience of this myself its just what he told me.
I have shot some really damp nitro cartridges, and they left the gun pretty fouled up and were slow shooting you had to seriously adjust your lead they still worked when you were on them but they were slow, i imagine the guy in the story above's nitro carts were just damp after years of soaking up moisture.
Black powder is similar in that its hygroscopic ( is that the word? ) i have cut old shells open and the black has been nearly a solid disk, i don't know how this would affect burn rate etc..
If your going to shoot them i would advise extreme caution, and follow best practice for miss fires. Pull the trigger, click, wait thirty seconds, then carefully open gun with the breech facing away from you and the barrels still pointed safe and open the breech, wait thirty seconds, then remove the cartridge and don't play with it until you are certain its safe.