We need you to use a Digital Caliper to measure the diamter of the undamaged midsection of the bullet's body in hundredths-of-an-inch. Also, we need to know whether it has a solid flat base or a cavity in the base.
Meanwhile, it appears to be a fired .50-70 Springfield Rifle bullet, made from 1866 into the 1880s. You can see a fired .50-70 Springfield bullet on the right in the photo below.
If you dug it in Mississppi it may have been fired by postwar yankee Occupation troops. However, many thousands of .50-70 Springfield rifles got sold as "military surplus" to the public for game-hunting after the US Army adopted the Model-1873 Springfield rifle. The other photo shows a casting-mold for making .50-70-450 bullets which was produced by Winchester in 1880 for owners of the military-surplus .50-70 Springfield rifles.