I wanted to add something to my earlier posting. This may relate better to the OP's original question?
There are two ACW carbines that are also .50 cal., The Maynard and Smith.
Several mold manufactures have produced molds for these two arms that work very well within the 56-50 Spencer's.
All three arms are used within the N-SSA and a good deal of info maybe found on their forum with a search.
Common bullet dia. maybe from .512 to .516 depending on the mold maker and the alloy one might be casting with. Bullet weight may also vary depending on maker and/or alloy. Smith bullets usually being shorter and lighter, with Maynard's usually being somewhat longer and heavier bullets.
It is important to note here that the Smith and Maynard's are both single shot breech loaders. Overall Cartridge Length is not quite as important as it will be within the Spencer Breech Loading "Repeater".
Overall Cartridge Length may need to established to work best in your specific Spencer firearm, Original or New made.
My best,
Blair