Just starting with a .32 Miller (so marked).  Bbl appears to be a former .32-40 highwall barrel, on a Dave Mos modified (rf to cf) BSA Int'l Mk II action/stock.  Came with chamber cast and a cylindrical die for forming brass.

Brass is easy to form, although .38 Spcl turns out to be just the right length and .357 needs much trimming.

Easy to form the brass - with a 4" C-clamp or easier and faster with a 1 ton arbor press.  I use a bit of lanolin based lube I made up for serious forming/re-sizing.  Annealed the first dozen, but got better results not annealing.  Might do so after sizing; will flip that coin later.

Bore appears to be .312-.320 give or take (used calipers on a buck-shot driven down the bore.
 
Building a breech seater is next and then:
a) what bullet?
b) what powder?
c) how much powder?