Nice, albeit imperfect, dug "French" bullet. This is the scarcer variety of the two more common French triangle base bullets. It looks a lot like a regular three ringer, but has the extremely unusual base cavity.

 

The previous owner of this bullet believed it to be a very early "eyeballed" battlefield pickup rather than something dug with a metal detector and shovel. He says it was part of an old collection in Berks County, Pennsylvania. The bullet (and several others) were labelled as being found on the Antietam Battlefield. The main thing though is that this is a decent middle to slightly above middle grade example of a semi-scarce bullet.