Even back in the day, I've been known to buy and store different ammo for future use.
All my ammo gets stored in a 30 or 50cal steel ammo can and inside a sealed military ECM shipping container with desiccant.
I also keep my powder and primers in the same shipping container - yeah, it's a big one!
Pretty much everything I have is still 100 percent.
There are two exceptions:
Old paper 12 and 20ga shells from before modern times....I don't shoot them just because.
Remington Golden Bullets packed in the 525 round bulk boxes.
The Remington Golden Bullets are about 50 percent and quite happy to not fire at all, even on 2nd, 3rd or 4th try.
I think it has to do with bulk packaging more than anything else.
The bullets can bump around and the seal between the lead projectile and brass case will break down allowing moisture to enter.
I'm sure this doesn't happen over night and must say these bullets have been stored since mid 1970's.