It is a civil war yankee 3-groove Minie bullet, which has been hard-rammed and fired. Hard-ramming with the rifle's ramrod made the circular indention at the bottom of the bullet's curved nose, and firing through a rifled gunbarrel made the ridges (called rifling-marks) running up the bullet's sides from its base.) As Tony In SC indicated, when you give us precise measurement of its diameter we'll be able to tell you what caliber it is. Going just by the photos, it looks like a .58-caliber.
If you find another one, please do not remove the greyish-white Lead Oxide patina, which most collectors consider to be the proof that the bullet is really from the civil war and not a modernday reproduction Minie bullet. (Yes, reproduction Minie Bullets are made and used for Blackpowder Rifle hunting and target shooting.)