Crankshaft,
I have encountered several .43 Spanish and .43 Spanish Reformado ("reformed") Rolling block rifles, including one Oviedo Arsenal sample
Various sources (Barne's Cartridges of the World, Layman's Remington Rolling Block MIlitary Rifles, etc) describe the Reformado as a later re-chambering of existing .43 Spanish RRB. Other sources claim the .43 Spanish Reformado came first, and the .43 Spanish came later.
I've dissected original .43 Spanish Reformado rounds, and the .454" diameter bullets have a thin bronze/guilding metal jackets filled with a soft lead core. If the Spanish considered it safe to fire 0.015" oversize jacketed slugs in a 0.439 bore, it was their business, not mine!
Suffice it to say that I've encountered RRB bore diameters that were from 0.439 to 0.455. The only way to positively identify the size bullet to use is to slug the bore and do a chamber cast. I would match bullet diameter to groove size, regardless of which cartridge it was chambered in.
In every instance, chamber neck dimensions were very generous. Case life can be short even if you necksize. This was supposedly a design to make the Remington Rolling Block more reliable under battle conditions.
YES, this caliber is very shootable as a BPCR, but it requires more work and load development because of the differing bore/bullet sizes.
HTH
John