Scientists are only as good as our sources and tools, so I respectfully refer you to "The Arms and Equipment in the Civil War Message Board" in this same series. I appealed to the "Arms and Equipment" forum on matters of firearms and ammunition for specific units with good results, especially from Union quartermaster records and other very good resources I didn't think of myself. Place your query about Spencer cartridges there with a list of the Federal military units involved and see if I am correct.

Bruce Nichols