The problem with a claimed Civil War association is that while there might be proof that the ancestor served in that war, that does not provide any proof that this rifle was carried in the war, by him or anyone else, or even belonged to him.
Since that is not a standard military rifle and probably is not even the standard .58 caliber, it is not very likely (close to impossible) that the rifle was carried in the war. (It would be like a soldier in Afghanistan carrying a Winchester High Wall in .32-40 - not impossible, but darned unlikely.)
BTW, set triggers were often used on hunting rifles, not just on target rifles.
Jim