The Barrel is no problem. The chamber reamer would have to be special made for .458 bullet as would the dies. Modern CIP standards for .577/450 cal for a .464 bullet, Modern reamers are made to CIP standards. Dies are made for standard CIP measurements and the larger original neck. .468-.472, not .458 bullets. You would need special order dies made. All available but not cheap.

This cartridge could never be an NE although you could load Nitro for black. It is to short and fat for that.

While I believe the Martini action is plenty strong, the rotary action of the breech block closing calls for a bit longer head space in chambers. No-go is .010 vs the recommended .006 in chambers for smokeless powder pressures.

If you set headspace when you chamber for .006 when building the gun you will encounter issues chambering rounds, even those that are full length sized.

Because of this longer head space you must use solid head brass in this chambering. If you use reformed shotgun brass there is very real danger of head separation or head blow-out.

Quickload. I have tried some of the suggested Martini loads. These are theoretical predictions. I would avoid them like the plague. They are scary. Those I tried shot very hard in the Martini. After shooting the brass had to be resized in a Lee die before it would go back in the chamber. Shoulders had to be set back and bodies reduced. I don't think brass life would be good.

The NfB loads using 4198 showed promise 10 years ago when I was working with them. All the notes on that experiment are long gone in some old computer and the write-up I made on Gunboards was lost in the server failure in 2006.

I can tell you it took a lot of work to come up with a load that worked. I definitely had to have a filler. I definitely had to have Magnum primers. But I did have a load that worked pretty good.

That is 100 yards using issue military sights.

I would also recommend that you use only a solid Greener Action. Do not use a split action for your project.

I myself try to avoid using the Greener action for anything, it is just two difficult to work with and I know what I am doing. The chamber in my 577 2-3/4" needs redone-reamed with a correct reamer. Simple hand ream job. I just hate disassembling and reassembling that Greener action it is a PITA.

Good luck with your project and keep us advised.