For me it was 401 Winchester Self-Loader. It took a while to track down an 80 yr old die set, then cut, expand and fire-form brass cased 7x39. And finally, to track down some .406 bullets. Oh, and some really ancient load data.
I also reload 348 Win and Win 45 Magnum.
I also reload 348 Win and Win 45 Magnum.