Moodyholler,

Lots of choices on rimfire cartridges; according to "Providence Tool Co. Military Arms" by Edward Hall, there were 8 of them, plus 3 proof loads.  However, the best chances for the carbine were the .45-45 Peabody, the .45-60 and the .45-70 rimfires.  All had rims in the .646" to .664" range, with head size .560-.566", about the same as the .50-70 Rimfire.  Case lengths were 1.357", 1.485" and 1.402", respectively.  Hall also lists a ".45-?" with a 1.613" case length.

The Peabody was also chambered in .41 Swiss rimfire, but a hasty perusal of Hall's book doesn't say anything about them chambering them for Spencer rounds.  It says most Carbines were serial numbered over 100,000.

The firing pin on my .43 Spanish Peabody rifle goes along the side of the breechblock and then doglegs into the center.  The rimfire pins had the side channel all the way straight through to the rim seat.  It would take a lot of welding, redrilling and reworking to get a centerfire conversion; probably not worth the effort.  Maybe that new outfit that's supposed to be making Peabody rifles can supply you with a new centerfire breechblock and firing pin, and the other parts you need.

Datig's "Cartridges for Collectors, Vol. 4" has a section on Peabody rimfires.  The text says that Peabody cartridges are not bottlenecked.  Instead, they have "tapered mouth crimps".  Peabody cases are quite different from the .56-46 Spencer.  Suggest a chamber cast if you are going to try and get the old thing banging away.

I am interested in the 43 spanish barrel. What condition is it in and how much do you want for it?
Regards Bill