It appears to have struck a tree or wooden board, or perhaps a clay embankment.
Noticing its size in comparison to the US quarter, and its wide "mushrooming" (which indicates it is made of soft, very-pure lead), and noting especially the wide cavity in its base... it appears to have the characteristics of a civil war era Minie-ball.
Fortunately, there is reliable data about the precisely-measured weight of many varieties of civil war bullets. Therefore, as long as a civil war bullet isn't missing some of its lead body, we've got a decent chance of figuring out its caliber, even when the bullet is smashed.
Please get your fired bullet weighed on a precise-weight scale, such as a Jeweler's scale, and tell us the weight. Either grams or grains or ounces will do. Just make sure the scale ins't set for Troy ounces.