The .577 x 2 3/4 Light Nitro will fit in both the MH and the GP actions IIRC.
There is a thread on these forums somewhere (years ago) discussing this chambering in the Martini, in which I posted a photo of the MH and GP actions side-by-side, and a few pics of the cartridge being chambered.
Will do a search for that old thread when I get a few moments spare!