Help identifying civil war finds

Although your big chuck of flat lead honestly doesn't look like the piece I found, their were artillery shells that had lead sabot (lead wrapped around the shell to expand and grip the rifling of the artillery piece when fired). When the shell explodes the lead sabot fragments. The piece I found was about the size of a table coaster and was slightly curved and still showed some of the rifling. Your piece could be something similar, or it could be a chunk of camp lead used to mold bullets. Or it could be a more modern chunk of lead from something else.