I had many thousands of rounds of CCI, Remington and Federal from the late 80's and 90's. Most of it was sealed in air tight plastic bags with the old Seal-A-Meal sealer.
After about 15 years the cases began to corrode on the outside, usually starting at the base. Most of it would still fire.
My theroy is that during priming, primer dust would get on the case bottoms and eventually corroded the cases.
Another thing that happened to the Federal is the bullet lube coating the bullets broke down into a granular white powder. As I recall the federal was the only one that did that.
Also, when fired, in a revolver especially, light loads could be heard.
I'm working on the last 10,000 rounds of 20,000+ rounds of Remington 22 Short I bought in the mid 1990s. Now when fired low power loads can often be heard.