Your .256 idea is still the best, as that ballistic niche is sorta empty these days and has a lot of practical application. Go for it; I love watching other loonies spend money.
SO do I!
Well, if it'll make a fellow 'fire member happy (you) I could always change course and find a donor Weatherby and chamber it in 25/378 and shoot "everything" from sooper zappers (85 gr special made titanium solids so they don't fly apart!) to #3 buckshot (0.25 in roundball) gallery loads.
Probably stick with the 77/256 idea though.
Have fun today.
Geno