WWII brass shotgun shells are collector's items. Sell them to an ammunition collector and buy some fresh ammo.
Serioisly though, I bought two WWII M1 carbine magazines and a web pouch from an estate sale, ammo dated 1944, with green corrosion on the brass (stored under less than ideal circumstances). I cleaned the magazines up (they were lightly rusted). The ammo had no collector's value, so I dumped it in a container of hot water and dish washing liquid, and scrubbed the green verdigris off, and dried them on the kitchen window sill. They all fired when I took them to the range (30 rounds of .30 M1 Carbine vintage 1944 fired about fifty years later).
I also bought a box of mixed box of .45 ACP which included thirty rounds of WWII steel cased ammo (not in an original box, with twenty rounds of modern brass cased .45 mixed in. I fired those steel cased rounds in a vintage military match (3 targets in 3 strings of ten rounds) and shot one of my better scores with my .45.
I have also gotten some British .303 of 1946 vintage with several duds, loooong hangfires (pull the trigger, click, one Mississippi, two Mississippi, KaBOOM!).
Is vintage ammo safe? It depends mostly on storage conditions.
ADDED: Back in the 1970s my stepdad and I shot some .30-06 ammo he had inherited from a relative in Remington semi-auto rifle. Some of it was 1920s vintage; it all fired, but not all rounds fully functioned the action.