As a deep desert dweler , here are two of my experiences .
1st...around the early 1980s I had some WW2 30-06 that shot wonderfully . Sold all my '06s , so I sold my ammo to a friend. About a year later , I picked up a Garandicon again , and bought the ammo back. Turns out he had stored in in a garage during the one summer he had it. Went from 2 MOA ammo to 12-16 MOA ammo just like that.
2nd...beginning in '84 I went into collecting WW2 USGI firearms and sold off most everything else , including about 20 various H&Ks . Around 2005-2006 I was cleaning out my '79 pickup and found in one of the many storage pockets a left over , loaded , H&K 91 mag. It had spent at least 20 years in the summer's heat baking in the cab. I gave it to a friend and we shot it off one day. Ammo functioned fine , grouped as good as you could want , and showed no signs of it's age.
So....you never know.
Chris