He had a Whitworth during the war - but naturally the Federals took it when they captured him along with Morgan at Buffington Island. After the war, his more wealthy sister "grub staked" him to a start, thus the Spencer. However, since it's fiction, I could easily have him trade the Spencer for a Sharps, which I admit would make some more sense as a long range shooter. I doubt the Whitworth ammunition was readily available on the frontier in the latter 1860s, so looking for a more common rifle. He's headed to Wyoming and Montana.