Bullets shouldn't be magnetic since they are made of brass, copper and lead, but I guess these ones had enough rusty ferrous material stuck to them that I pulled them out with a magnet anyway.
The little rod below the horseshoe is also entirely made of copper and not all magnetic, but it was held on to the magnet by some rusty scrap.
I also got a fair amount of loose change, and what I think is a door knob that I was a little worried could be a grenade when I pulled it out amongst the bullets.
You do get steel cased rounds, and I'm pretty sure a lot were made as part of war measures (brass being comparatively expensive.)
I'm wondering if that rod under the horseshoe might be the remains of a plug from a telephone switchboard.
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.