Civilian ammo from the 50's and 60's is still 100% reliable. My dad had a pile of old CIL and Dominion 22lr, paper shotshells, and 30-30 from his father that were stored in a dry basement and still worked fine in the early 2000's. They were in the yellow boxes with blue markings. I kinda wish my dad hadn't shot them but to him it was just ammo. I think he still has some CIL SSG (00 buck I think) shotshells kicking around that he kept. He said he wont use them since he now knows they have some collectors value and I handload for him so he isn't short on ammo ever.
I regularly see younger guys at the range firing off old boxes of CIL, Imperial, Dominion, and IVI ammo that looks like it's from the 50's, 60's, and 70's. I don't know if it's inherited or from a gun show.