Parabola - thank you for this reply and the technical case details within, that is again very helpful.
I've noted in pics of the rifle that there are some fired .500 express cases there. I wondered (we'll see when it arrives) if the previous owner has shortened these to 2.5" to reload.
As for NSW guards having double rifles, two things struck me, the first round...followed by the second! Certainly a strong message to the inmates!

Let he who casts the first stone - be accurate.
If he cannot be accurate, he should practice first.