Luke,

The 600OK is its own case. Kind of looks like a scaled up version of the 458 Win.

There are a couple of blokes in Melbourne that have 585 Nyatis on CZs and all is OK with the single stack magazines. One of them posts as PC on accuratereloading.com and Australian Hunting. They are using the 650 grain Woodleigh Black Powder Express bullet loaded to about 2300.

http://www.australianhunting.net/index.php

Another calibre that might interest you is the 500 A2. That is basically the 460 Wby necked up to 510. I think most people would agree that the 500 A Square offers the best combination of power and practicality.