Back in the 60s when I owned a .270 and shot exclusively IMR 4831 (military surplus) for .50/pound...yes, fifty cents!!! I bought the powder from a neighbor that had bought a big lot of it...it was cheap and he may have bought a hundred pound keg.....I don' t know how large a lot he had.

He told me about 15 years ago that he had the balance of about 20 pounds in his basement and it was errantly forgotten to have the cap properly affixed. When he investigated it, he said it smelled bad and it " LOOKED" wet.

He very carefully discarded the powder to fertilize the garden.

I too had a can of half full powder that sat in the garage for twenty years going thru freeze and thaw and hot and all seasons in Southern Minnesota....it looked wet when I looked at it...and I very carefully burned it.

Powder kept in a " full" cartridge is sealed for all purposes with very little air space for Oxygen...I think it last a lot longer under those conditions.

Clearly powder can and will deteriorate in about twenty years if it' s not stored properly.

Possibly others can post more data than this.