Dang man! It's hard to tell esp. from a picture.
Soft lead can take all kinds of crazy forms (Esp, when fired as a bullet).
Your piece has good patina on it, so it's my guess that it's pretty old. ( I'm no M&M or Round ball to Rim-Fire expert)
but it seems to [maybe once have been round] also (on the inside-IMO has the look of HI Velocity impact) but that flat part gets me unless it struck something flat( (DAAAA) and it was once round or some other shape.
I,m getting confused myself. It could be about anything but you know where you found it and the areas history "I assume"
So good luck on the id .
If you don't already know there will be a Civil War Show in Dalton I think one of the first two weekends in Feb & if youve had no id before then take it there and get some opinions and enjoy the show.
Davers