[Travis McGee]

I have shot WW2 surplus .45 in nickel plated steel cases that was 40 years+ old, and it all fired. I have shot tons of 20+ year old surplus 5.56mm, it all worked.

[RightWhale]

20 year old better work, even 30 year old, I still have some in the box. Actually, I have no experience with old ammo failing to fire, but maybe 40 years isn't old. It's just that one box of 45 ACP from the 40s looked so bad I pulled the bullets and put them in the tumbler with the casings and then reloaded the sparklingly cleaned brass. I forget how I dealt with the primers, I'm thinking woodstove, but they got replaced, too.

[B4Ranch]

I've been told that a constant cool temperature is the trick to keeping old ammo good.