Mystery shell.

[andwaahs]

So I was in the garage to my neighbour and we had a look at a few relics he had, but this shell was a real conversation stopper.
It seems to have been fired, but is incredebly heavy. I unscrewed the small brass cap off the tip and there was a small firing pin and a detonator cap! no idea how to tell if this was inert.we removed them however.
Anyone any ideas on this one?