It is still a bit difficult to tell the measurements without using calipers but i am guessing it could be a .54 caliber minie. I am assuming that the cavity might be a cone cavity. It has been rammed and I can see rifling, so it is definitely fired. Commonly used rifles of .54 would have been the US M1841 and the Austrian Lorenz. I'm leaning toward it having been fired from an Austrian M1854 Lorenz if it is .54.
If you are getting into bullet collecting, an inexpensive reference to the many different bullets of the period I would recommend is A Handbook of Civil War Bullets & Cartridges by James and Dean Thomas. Beware, Civil War bullet collecting can be habit forming.