Although your bullet is unusually heavily corroded... I agree with Tom Buckley, its flat-bottomed grooves, little flat "cap" on the nose, and deep "false cone-&-pin" cavity mean it is a civil war era US .58-caliber Williams "Regulation" minie-ball. It appears to be an unfired one. Finding an unfired bullet often means there are (or were) several more of them nearby.
Regards,
Pete