.577 2 1/2” Express (yes, it did exist, was 1870’s intermediary between the Snider and the 2 3/4” and 3”).
Will fit a Martini, and 24ga Magtec bass just needs a trip into a .577 sizing die. Much easier and less expensive than fooling with .577 3” brass and having to cut to 2 3/4 or 2 1/2 to weasel into a Martini.
110-120 grains of black and 460-520 grain bullet.