Handing them in to the police would probably be a good idea, although the Bobby I spoke to on the day completely ignored me when I told him that I'd been pulling bullets out all day.
My colleague suggested I chuck them in a fire and see what happens but I don't think I'll be taking that advice haha
The plug from a telephone switchboard is a really good suggestion. One end of it moves in and out and it has what looks like a terminal for a wire in.
The outside of at least some of the casings is definitely brass judging from the colour, but some are so corroded I wouldn't be surprised if they were steel cased.