I don't know about the 8th Cavalry, but the 12th Missouri Cavalry didn't get their Spencer's rifles until June 1865 when they were outfitting for the Powder River expedition. They had been approved for Spencer's as early as December 1864, so it took a while to get them. Until then, they used the Sharp's carbines; their commanding officer detailed problems with the Sharp's as early as September 1864.

Tom Jones