Is there a solitaire version of Bang?
The Unofficial Bang! Blog has, among other great variations, a set of instructions (pdf) for single player Bang!

The main changes in this solitaire version are that all roles and cards held are known to the player from the beginning, and that you play as both the sheriff and the deputy, with other "NPCs" making semi-predetermined moves.

You can increase the difficulty by giving more cards to NPCs at the start, but not too many! The author can barely survive with two cards given to each NPC at start. (I'm afraid even I'm not that good :p)

Not always having a large crowd to Bang with, I have thoroughly enjoyed this rule set and have now almost played it more than "normal" Bang!