H4831SC is actually AR2213SC made in Australia and re-branded as Hodgdon H4831SC powder for the American market.

The reloading manual available online for the Australian powders does not list loadings for the 505 Gibbs but does so for the 500 Jeffery using a 500 bullet with 96.0grs AR2206 powder for a MV of 2300fps (45" barrel!!!). This is the only loading they publish for the Jeffery cartridge with AR2206 sitting between IMR3031 and IMR4895 burning rates.

Perhaps the faster powders are better suited for these large bottle neck high intensity (relatively speaking) cartridges rather than the slow H4831/AR2213?