Before attempting to give you the benefit (or not) of my own experience with a .56-56 Spencer, I need to ask a few questions.
First. what kind of accuracy are you getting? At what range are you shooting? What alloy are you using to cast your bullets? What is the hardness? What kind of bullet lube are you using? Do your heel bullets have lube groove ahead of the heel, or is the bullet coated with lube? Have you "slugged" the barrel to determine groove diameter? I think you will find M1860 Spencer carbines (if that is what your gun is), have tapered barrels. Mine measured .545" just ahead of the chamber and .535" at the muzzle! How are your crimping the cartridge mouth into the bullets, if at all?
I got my best results (3-3/8" seven-round groups at 50 yds) using IMR4227 smokeless powder with almost a full case, 18.6 gr, with a 411 gr. bullet cast from Lyman #2 equivalent alloy (9 lbs wheelweights + 1 lb 50/50 lead-tin bar solder). Although I tried a number of other bullets, from Rapine Bullet Moulds (now out of business...Ray retired years ago), my best results were from a hollowbased Lyman #433476AX mould, sized .535-.538" dia. This is an inside lubed bullet. My brass consisted of Dixie Gun Works .50-70 cases, that were cut down to 1.000" +- .002", resized in a standard .50-70 die, and then inside neck reamed to .525" using a custom reamer, slightly longer than the length of the seated portion of the bullet. I also annealed the brass approximately half its length at the neck. This produced a case wall thickness of 0.0125-0.0135" at the mouth. Crinping was done using a .56-56 crimping die from C&H.
Interestingly, the .56-56 is proportionately like .45 LC. Water capacity of the cases is about 22.5 gr of water. Relative Sectional Density of the bullets ran around .196-.205, depending on bullet weight. (RSD = bullet wt (grains) x .0000499) for a .535" groove)
of the BP substitutes were I shooting this 157 year old gun today. I did use Pyrodex "P", with some loads, but accuracy was fair to poor.
Let me note that the Spencer carbine was NOT noted for its accuracy, partly because of the ammo variations from various manufacturers, and partly due to the slow twist (1 turn in 52").
Not sure if this will be of help, but perhaps it will give you some clues.