I don't have the book here with me right now, but I believe the author Ned Roberts who wrote the Muzzleloading Cap Lock Rifle (a definitive work on this area of muzzleloading) talked about a deliberate shot taken during the civil war where a confederate officer (perhaps a general) was killed at a distance exceeding one mile. Also there is a book on Hiram Berdans sharp shooting regiment detailing that many of these men were using there own rifles before the percussion sharps became available for issue. These rifles were quite often target rifles that were pressed into service and many target rifles in the 1850's were of the underhammer principle.
One book that I do have with me at the moment is a collection of stories by Lucian Cary who used to write short stories for the Saturday Evening Post. Most of Cary's stories centered around a character named J.M. Pyne who was a loosely disguised Harry Pope, the late great barrelmaker, who was a good friend of Cary's. In any event one of the best stories in this book is called the "Madman of Gaylords Corner", this details the use of a large bench underhammer rifle and a mad sniper who uses it to make a 1000 yard shot to avenge the death of his son. The name of the book is "The J.M. Pyne Stories & Other Selected Writings by Lucian Cary. I bought my copy through Dixie Gun Works, you can probably order a copy from there or from Barnes & Noble. It was compiled, published, and edited by.
Guy Lautard
2570 Rosebery Avenue
West Vancouver, B.C.
Canada V7V 2Z9
Hope this helps