.450 BM in an African is ridiculous.
Some of the suggestions in this thread are ridiculous, other suggestions have already been produced several times over.
In this scenario, I don't think the mass market should be considered, if it is an order of 100 units or less, consider a passionate niche market... like big bore dangerous game units. Coupling the Ruger fans with an option no longer available except in high demand/high priced used units is the best way to go. The RSM's were offered in .458 WM, now North of $3-4K, and the old 77 tanger in .458 WM for North of $2-3K, I think an African in .458 WM would sell out 100 units in the blink of an eye... bedded, cross-bolted, barrel banded, express sights, 22-24" barrel... GONE.