223 is near perfect. .256 is awesome but it’s a pain to form and factory brass is laughably hard to come by, and expensive when you do.
if you go that route... take a POS 30-30 sizing die to a chop saw to make a mid-way forming die. Anneal, size to 30, anneal, final size, inspect for cracked necks, trim, load. 30-30 forming dies are easy to make the midway former and a lot cheaper than a $200 set of forming dies that do the same thing.... but still, avoid .256 unless you have some odd fascination or sentimental tie to the cartridge... like your dad getting one (marlin 62) for his 13th birthday and teaching you to deer hunt, shoot using a scope, and how to reload with it some 30 years later...