The cartridge is uncommon in either guise, the Magnum or the No.2, but neither could be called rare. Several board members here own or have owned rifles so-chambered, me included. Read this thread for details of mine.

Both cartridges were loaded 'nitro for black', however only the larger 3 1/8 inch case was loaded full nitro briefly before tossing the towel to the .500 x 3-inch NE. Both Fleming and Hoyem are far better references than COTW BTW!!

Either version of the .577/.500 cartridge would be a fine contender for a shotgun conversion, however the straight-case .500 would be much easier to build and load for.