Cool story.....
Ammo will last a surprisingly long time but not in those conditions.
I had some 7.65 Argentine Mauser ammo made by FN in 1933. It generally fired OK but there was an occasional hang fire. Not a long one but still not instant ignition.
Left out in the elements like that, it would go bad pretty soon.
I’ve found 45-70 casings and cartridges in the high Sierra where the casings were nice but the cartridge I found was rotted open, probably just a hint of moisture need get in to powder to roach them.
I’d be climate and them being in the tube there though would give any loaded in it still being fire able, all be it maybe just a fizzle.