2.565" from the breech face to .765" area that enters the forcing cone and almost exactly 3" from the breech face until the full rifling diameter of .695". I have a local source for .678" and .662" round balls. I shot it yesterday with the .662" balls and a .020" patch. It sounds like a good place to start with the proper case. I thought I read that your had good success in one of your guns with round balls a couple thousandths of inch over the groove depth. All fun stuff.
Steve