It is definitey a fired bullet which "mushroomed" outward due to a very hard impact on a solid object. But "mushroomed" fired bullets can be very difficult to accurately date, due to the extreme damage. About the only thing which might tell us whether your bullet is from the 2nd half of the 1800s or the 20th-Century is the precise measurement of its still-circular flat base (in the center of the "mushroom"). If its base is any less than .5-inch in diameter, it is most likely from the 20th-Century.