RAN Lieutenant's rank slide

We were in Canberra one night, and my Mum got talking with a man who turned out to be a submariner in the Royal Australian Navy. He gave me his rank slides as a gift (he'd recently been promoted, I think), and I gave one to my brother.
Bullet of unknown calibre

I bought this at a museum. I put the ruler next to it because I'd like to know its calibre.