Attached is a photo of original rimfire Spencer rounds next to thier modern centerfire conversion counterparts.
56-50:
On the left is a Taylor's 56-50 with .512 Modified LEE 350 grain bullet in a Starline case. Next to it is a military 56-50 rimfire with no headstamp
Besides the orignal being rimfire and copper, it is slightly longer and heavily crimped over the ogive of a .515 to .520 diameter bullet to insure maximum combustion (Christopher Spencer thought this was too much crimp, and introduced his 56-52 rimfire with a healed bullet like the 56-56. It is interchangable with the Military 56-50)
.56-56:
On the far right is a modern centerfire .56-.56. It loaded with 4-D dies and uses a Buffalo Arms modified 50-70 case cut to just over an inch (about 1.012"). It is loaded with the Rapine 370 grain, .535 diameter healed bullet (.512 heal), outside lubed (like 22 rimfire) with Alox lube on the exterior of the bullet, though I rubbed off most for the picture.
Next to it is an original Civil War vintage .56-56 rimfire (which was actually designated as "No. 56 cartridge for Army and Navy Spencer repeaters" The number .56 refered to the case diameter at both base and mouth. It was actually nominally .54 caliber, with bullets I have checked ranging from .536 to .542. You will notice it is copper, and only .85 inch with a 375 grain outside lubed bullet.
The reason it has a shorter case than the modern cases is that its bullet had a very short heal with a deep groove that the very mouth of the copper case crimped tightly into. The Modern Rapine bullet ( RAP 535370) has a longer heal than the original, so BAC makes the cases .2 inch longer to help grip this longer heal. Modern brass cases can not form the same way copper did, and rely on a shallow taper crimp.
With the .56-56, I find that very hard cast .535 bullets (I use wheel weight lead with some linotype added) stay crimped better and lead the barrel far less. I tried pure lead, but the crimp would not hold (especially with heavily compressed loads), and they tended to come apart feeding in some original Spencers.
Note: GAD custom Cartridges offers original length .85 inch case, though these must be used with the Rapine 546375 mould that has a shorter heal. You would need to cut down the 4-D crimp die to use the shorter GAD casess, as 4-D is regulated for the BAC 1.12 cases.