I still have some H 4831 that I bought in the middle sixties (for 52 cents a pound!) and it still works just fine. I live in a dry climate (15 inches total precip per year) and the powder has always been stored in metal cans in a closet in an unheated building. Gunpowder, as you probably know, is quite stable chemically, and under good storage conditions will last longer than you will.