Relicmeister, I was put on the right track about it because long ago I got to handle and examine the results of a yankee ammo-supply wagon that the Confederate cavalry burned on a raid. That wagon contained hundreds of .52-caliber Spencer cartridges. The exploded copper Spencer casings looked basicly the same as your find -- a thin cylindrical metal that was torn open and "bulged" extra-wide by explosive blast from the inside. They were so cool-looking that even after 30 years I haven't forgotten their exact appearance. So, when I saw your find, I said "Aha!"