It appears to be the cut-off bottom of a civil war yankee .54-caliber Minie-ball (whose original diameter was about .525-inch, but yours is bent out-of-round. Most had three grooves encircling the bullet's body, and a wide cone-shaped cavity in the base... which accounts for the tapering hole in the object you found.
Usually, excavated 150-year-old lead has a white-ish/grey-ish color on its surface. Did yours come out of the ground looking dark lead-colored, or did you scrub off all the white-ish/grey-ish oxide?