My .42 Berdan uses the Spanish case or a reformed .348 Winchester. Like the previous, I also only use BP and am thrilled with the accuracy, smoke and smell of shooting. I shot a box of 20 yesterday (with my son and grand daughter also shooting it.)
If you want to try BP, I can get you info and pointers. Dies are inexpensive - if you neck size only. I use .44special for nexk sizing and seating.
My records show a 370 grain commercial hard cast lead round nose projectile.
Goex ffg powder at 67 grains with an Ox Yoke "wonder wad" over the powder (this was added after the first shooting session to help keep the powder residue soft), standard large rifle primer.
Cases made from .348 Winchester brass.
The brass was converted using a set of expander plugs made from standard long shank machine bolts of progressively larger sizes with the threaded section cut off, shanks tapered and polished smooth.
It was then trimmed to length and turned to size including rim diameter and thickness.
It was finished off in a set of Lee dies.
I used a bench mounted arbor press to expand the brass with the bolt expanders, I used RCBS lube on the dies to ease the expansion and die removal.
During the forming process the cases were annealed.
This weight of powder with the wad resulted in a slightly compressed load in this batch of brass.
I never felt the need to up the load as with this projectile it shoots (more or less) to point of aim at 100 yards, also, given the vintage of the Remington and its overall condition I feel no need to push the envelope.
This projectile is really to hard for this use but I have no inclination to take up casting so I'll just live with it.
Understand that this firearm is not a "daily driver" but was just a "let's see if we can do it project".
In the end we did get it done and made it work.