First, a tiny little CAVEAT: I don't own a Spencer. I have never own'd a Spencer. I have never shot a Spencer. I have no desire own, shoot, nor load for a Spencer. In fact, I have never, in person, held nor fondled a 56-50 cartridge. SO:
Obviously, I have absolutely no pearls of wisdom to add to this thread. Nothing. Nada. So ... if all desire to ignore it, I Shant feel at all slighted. Nor offended. HOWEVER:
The 56-50 and the Spencer were designed and developed way back inna wabac (Sherman and the Professor here) when Chinese powder was king (Black Powder) and they were intended for Chinese Powder. Ergo, my load recommendation would be just enough of your favorite Chinese powder to provide about 1/16th inch compression. Or a similar amount of either APP or Black MZ to just kiss the base of your favorite loaded projectile. Those suggestions will do really nasty things to whatever it is you shoot your Knew Two Ewe Carbine at.