The 38-40 is an old and not much seen caliber, nothing wrong with it though. Brass is available from Starline and it takes 10mm/40 bullets with no issues. I use Lee 175 grain cast on top of 8 grains of Unique in everything except old BP Colts and Winchesters like the 1878 and 1873, it's my go to load in the New Service and 1892. It's accurate with both BP and smokeless.
I have a Ruger Vaquero in .38-40 with a spare .40 S&W cylinder.
maybe have a couple dozen rounds through the .40 cylinder, the cool factor of the old “dash” calibers wins every time.