I'm not sure about shotgun ammo due to its plastic components. Anybody know?
Older shotgun ammo had brass casings, the plastic casings are a relatively recent development. The big question for a shotgun shell is that one end by definition isn’t sealed all that well and therefore is susceptible to moisture ingress. I don’t recall anyone finding old shotgun ammo and testing it, though.
Just checked a couple of sources. Seems that some shotgun shells had paper hulls - those are even more prone to become duds.
However, if the shotgun shells are stored properly in a cool and dry place, they can apparently last at least 50-60 years if not more.
I’ve shot paper hull shotshells from the 30’s and most of it functioned fine. About a 10% failure rate for the stuff I had. The plastic stuff should hold up better, doesn’t attract moisture the way the paper hulls and fiber wads did.
Modern shotgun ammo should do about as well as the centerfire rifle and pistol stuff. I've seen the old paper-hulled stuff with fiber wads go bad due to moisture absorption, but plastic shotcups/wads and shells create as tight a seal against moisture as a crimped brass case.
I inherited my grandfathers old model 97 Winchester with about 500 rounds of ancient (40’s) Winchester 12 gauge shells. They are all paper and cardboard, no plastic, and I have yet to have a missfire with the stuff yet. (I shoot it because I don’t trust the newer rounds in that old a gun.)