A number of years ago, I developed smokeless loads for a M1860 Spencer Carbine, cal. .56-56. (Actual groove diameter tapered from .545 at the breech to .535" at the muzzle.) Using a 380gr. Rapine #546390 mould cast with #2 alloy (9lbs wheelweights+1 lb 50/50 bar solder), behind 8.8 gr. Unique gave me a muzzle velocity of 959 ft/sec and 5 rd groups of 3-1/4" at 50 yards. Brass was shortened .50-70 cases. Some other reasonably accurate loads were 18.6 gr of IMR4227 behind a 411 gr. Lyman #533476AX hollow-based bullet.
The interesting thing I found, was that the Relative Sectional Densities and water capacity was similar to the heavier bullets in .45LC. The 380 gr. bullet mentioned above would be equivalent to a 263 gr. .45 bullet. RSD = w (gr)/7000 x dia. x dia. Water capacity of the cases was very similar, depending on the seating depth of the bullets, etc.
I am NOT recommending these loads for use in an original Spencer, and anyone doing so does it at their own risk. Out of respect for the 150+ year old gun, I have quit shooting it altogether, but for .56-50 replicas, .45LC might be a reasonable starting point for smokeless loads.