The bullet's basic ID is an unfired civil war yankee-made .58-caliber Minie bullet for use in a Springfield Rifle. It is commonly called a "3-ringer" but actually what we see are grooves (indented), not rings (which project out from a surface, like a ring on a finger). It has been "modified" for some other purpose, most likely as a fishing-weight, which Callicles and Chiltepin have already suggested.
For certainty, though, please clean the dirt off it and post some additional closeup photos, shot from different angles.
I'm curious about what is projecting from the bullet's nose. Is the loop in the bullet's base made of lead, or of iron? I'm thinking maybe the nose projection is the end of an iron square-nail which was driven through the bullet, and the nail's other end was bent over to form a loop. (Soldiers with a need in the field can be amazingly inventive at constructiing what is needed.)