What is the mould situation like in .500"?

That modification in bullet diameter IS interesting, but the 2.1" .50 Alaskan will work in the Martini & there already are a lot of bullet designs for it, and much cheaper than jacketed bullets, yet in that large a bore, arguably just as effective on game, due to the variations allowed through hardening and tempering.

There are a number of great ones in .510", from 300gr. through 700gr.

The .50 Alaskan can use AlL of them for full power loads

I used to shoot a 550gr. RCBS bullet and 520gr. Lyman cast in my Rolling Block .50 Alaskan and my brother is currently shooting a 600gr. Buffalo Arms cast bullets in his .50 Alaskan M74 Sharps.

His 3 shot groups are running under MOA (just over 1" at 100 meters), however a 5 shot group strays out to 1 1/2" - at 100 meters. His 10 shot groups are still in the 1 1/2" range - and that is using black powder and aperture sights, not a scope, and from an 1874-designed rifle.

A 600gr. from the .50 Alaskan at about 1,700fps might work well on just about ANYTHING.