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 got 1000 rounds of .303 British made in 1918 and the 400 or so I’ve fired have all been perfect. The Enfield needs sore cleaning after a batch of them though!