A test: get a piece of bare steel, sheet metal preferably. Clean all rust, oils, dirt, etc off of it.
Pull a bullet from it's case and dump the powder out.
Hold the case mouth up to the clean spot on the sheet metal and have someone tap (set off/detonate/whatever) the primer with a nail and a hammer. You may want to hold the case with some channel-locks or something.
Put your metal away where it won't be disturbed for a day or two.
Check your primer spot for rust.
We used to do this in the shop at work when I worked nights. If you get several vintages of bullet you will see different amounts of rust as some were more corrosive than others.
