Appears to be a Model 1863 U. S. Rifle Musket. Cannot see the lockplate markings but it could have been made the U. S. Armory at Springfield or by one of the private contractors (Jenks, Mason, Norris & Clement, Remington or Savage).
Edit - Correct, CWDF. I see you have played the trivia game before, though it was quite a while ago.
Hope you'll come back and play again.
Hoosier