An Austrian gunsmith named Pfeifer makes a single action wheelgun in .600 Nitro Express. It's called the Pfeifer Zeliska Revolver. I have an article on it as a PDF, if anyone is interested. It's in German, since it comes out fo a Swiss gun rag, but it has lots of nice pictures.
The ballistics for factory .600 Nitro Express out of the Pfeifer-Zeliska are 900 grains at 1515 fps, for 4586 fpe.