Copper cased ammo?

[t1066]

Gonna make a guess. Reloads, and someone soaked the cases in vinegar or acetic acid. Too long in too strong an acid will leach the copper out of the brass, weakening the brass and discoloring it. Do I have that right?

Almost. It leaches the zinc out of the brass, leaving the copper.

Something like this: https://corrosion-doctors.org/Forms-...zinc-valve.htm

[Bang]

There you go. Kept zinc sacrificial anodes on parts of the boat to protect them from reaction in salt water.