Though a non-magnetic shell could be made out of an austenitic steel (like manganese steel).Try using a magnet on the shell, if it sticks its steel.
The magnet didn't attract to the shell at all or just not strongly enough to stay attached? I should think the former case would rule out both stainless (chromium) steel and nickel-plated steel. The drum couldn't be chrome over brass, could it?That looks like the drum. I put a magnet on it and it would not stick. I think that means it's stainless steel. I believe the made that drum in steel, stainless and nickel plated steel.