I don't know how much the Spencer was used in the Civil War but it saw notable action in the late 1860's on the frontier. This fellow carried one:
http://en.wikipedia.org/wiki/Roman_Nose It was used extensively in this desperate battle:
http://en.wikipedia.org/wiki/Battle_of_Beecher_Island There were some spent Spencer cartridge cases recovered from the site of the Little Bighorn battlefield.