As long as Black Powder does not get "Wet" it will essentially last forever. Smokeless is subject to deterioration, but it gets weaker, not stronger. What is most apt to fail in older shells is the primers. This can result in a mis-fire or hang-fire.
As several have warned "Watch Those Bores", you don't want anything left in them before firing another shot. Have "Never Ever" heard of a shell getting stronger & blowing up a gun just because it was old. Shells don't get Stronger as they deteriorate, they get weaker, thus the risk of something being left behind in the bore.
