I believe it is a 500 Jeffery necked up to .584. 500 grains has some 900 grain bullets for it that would be interesting. The rifle will probably be loaded to 1900fps with 750 grain bullets. Same as the .577 2 3/4 loading. That is plenty for anything in Africa for sure.

POI
I shot it once only without the muzzle break. The man who owns it only had two rounds left after sighting in. It is really not as bad as one would assume. A very hard push but nothing like a 460 WB. Much more like a 505 Gibbs.